Create an Alphabet Song

Bash (+coreutils), 81, 87, 82, 78 bytes

Uses the man page for X, as the source of words.

Golfed

man xyst\  x|&grep -Po '\b[a-z]{4,} '|sed 's/\(.\)/\u\1 is for &/'|sort -uk1,1

EDITS

• Used a non-existing 'xyst' man page + |& to save 5 bytes;
• Saved 4 more bytes, by swapping sed and sort.

Test

%man xyst\  x|&grep -Po '\b[a-z]{4,} '|sed 's/\(.\)/\u\1 is for &/'|sort -uk1,1

A is for also 
B is for build 
C is for computing 
D is for distribution 
E is for entry 
F is for following 
G is for graphics 
H is for hierarchical 
I is for implementations 
J is for just 
K is for keyboard 
L is for listing 
M is for manual 
N is for network 
O is for output 
P is for programs 
Q is for quite 
R is for runs 
S is for system 
T is for transparent 
U is for used 
V is for various 
W is for window 
X is for xyst 
Y is for your 
Z is for zeros 

Python 2, 88 77 bytes

-11 bytes thanks to xnor (avoid the zip by traversing the string and counting c up from 65)

c=65
for x in'niooaauusoioaiuaaoiineeaei':print'%c is for %c%st'%(c,c,x);c+=1

Try it online!

(A port of my Jelly answer, when it was 56 bytes.)

A is for Ant
B is for Bit
C is for Cot
D is for Dot
E is for Eat
F is for Fat
G is for Gut
H is for Hut
I is for Ist
J is for Jot
K is for Kit
L is for Lot
M is for Mat
N is for Nit
O is for Out
P is for Pat
Q is for Qat
R is for Rot
S is for Sit
T is for Tit
U is for Unt
V is for Vet
W is for Wet
X is for Xat
Y is for Yet
Z is for Zit

Bash, 78, 69 bytes

Aardvarks, Babushkas and Kamikazes !

Golfed

sed -nr '/^[a-z]{9}$/s/(.)/\u\1 is for &/p'</u*/*/*/words|sort -uk1,1

EDITS

• Got rid of grep, -9 bytes

Test

%sed -nr '/^[a-z]{9}$/s/(.)/\u\1 is for &/p'</u*/*/*/words|sort -uk1,1

A is for aardvarks
B is for babushkas
C is for cablecast
D is for dachshund
E is for eagerness
F is for fabricate
G is for gabardine
H is for habitable
I is for ibuprofen
J is for jabberers
K is for kamikazes
L is for labelling
M is for macaronis
N is for nailbrush
O is for obedience
P is for pacemaker
Q is for quadrants
R is for rabbinate
S is for sabotaged
T is for tableland
U is for ulcerated
V is for vacancies
W is for wackiness
X is for xylophone
Y is for yachtsman
Z is for zealously

Makes use of /usr/share/dict/words:

words is a standard file on all Unix and Unix-like operating systems, and is simply a newline-delimited list of dictionary words. It is used, for instance, by spell-checking programs.