The song hinted at by the puzzle is One, which features the refrain, βOne is the loneliest number that youβll ever do.β The 5 theme answers are as follows:
- 13A, 28A, 23A β Three Dog Night β The late 60βs/early 70βs band that popularized (but did not write!) the song One.
- 1A β Magnolia β The soundtrack to this 1999 Paul Thomas Anderson movie prominently featured a version of One performed by Aimee Mann, first recorded in 1995 for a tribute album for the author of the songβ¦
- 20D β Aerial β One was written and first performed by Harry Nillson on his album Aerial Ballet.
For no other reason than I heard it this week and thought it was interesting/hilarious, here is Paul F. Tompkins talking about attending a script reading for Magnolia.
All of the answers, with explanations:

1A β Magnolia β I + long(yearn) + am backwards (against the grain) + a
5A β ClichΓ© β CLI (Just over 150 β 151 in roman numerals) + che (niche endings)
10A β Xia β X(unknown) + I + a (admit at first)
11A β True or False β dishonest, or anagrammed, βLeRue sat forβ
12A β Piecemeal β Pie (baked good) + cemeal (cereal (cornflakes) with a new filling: a drop of milk (m) replacing the r)
13A β Three β hr (Human Resources) + tee (simple outerwear)
14A β Gazebo β Gaze (stare lovingly) + Bo (Bo Derek)
15A β Torment β men (dudes) amid tort (legal trouble)
19A β Grandpa β R (republican) in (middling) GA (Georgia) + ND (North Dakota) + PA (Pennsylvania)
21A β Sculpt β Scul (sound of mind, or skull) + PT (Barnum)
23A β Night β Nigh (close to) + t (the beginning)
26A β Reach into β Re (note) + a + chi (life force) + nto (not uncivilized, or anagrammed)
27A β Infestation β In + fe (iron) + station (train depot)
28A β Dog β D (five hundred) + o (or half) + g (1000)
29A β Seemed β seamed (stitched by the sound of it)
30A β Lapdesks β LAPD (Southern California cops) + esks (start to eat, swear, kibbitz, sometimes)
1D β Mixups β MX (missile) + I (solitary) + UPS (delivery company)
2D β Guatemala β Tea poured (anagrammed) into mug uprightβ Guatem + a la (as is done)
3D β On the mend β on (over) t (last of strict) + hem, end (limits)
4D β In utero β within mINUTE RObot
6D β Lo-fat β Loft (spacious floor-through apartment) + a
7D β Color β Col (lock up, strike (k) out) + or
8D β Elements β said Elle (French woman) + laments (bemoans)
9D β Goblet β Gob (old seadog) + let (commissioned)
16D β Roughened β Ghee (clarified butter) + Round (shell)
17D β Empanadas β Em (Dorothyβs aunt) + pa (father)+ nada (Jack, or nothing, in Spanish) + s (start to sample)
18D β Agencies β βcan siegeβ diabolically, or anagrammed
20D β Aerial β Ariel (The Little Mermaid) in the sound
21D β Stamina β Animates (brings to life, almost β i.e., without the e) upside down (doing a handstand)
22D β Forges β Forages – a (scours for material lacking a)
24D β Gaffe β Gaffer β r (cutting off the techβ¦at the base)
25D β Taste β within fashionisTA STEreotypically