Thanks for the great suggestions so far. Here’s an update. Still looking for an X …
A - second-rate, airhead
B - bump-start, butthead, bighead
C - cooktop, crackhead, clubhead, reactor core
D - godsend (-> god’s end), Land’s End, head-butt, rearguard
E - sweetheart, centrepiece, dovetail, endgame
F - offending
G - midnight, legend
H - hothead
I - midpoint
J - jump-start
K - weekend, cocktail, bookend
L - midfielder
M - masthead
N - nightcap, detention centre
O - photo finish
P - apple core, midshipman
Q - leading question, earthquake’s epicentre
R - redhead, Centre Parks
S - headstrong
T - third-rate, butt-end, midwestern
U - upstart, community centre
V - private eye?
W - centre-forward
X -
Y - ponytail, happy ending, soggy bottom
Z - midsized