Peg's short of teeth! (3)
I believe the answer is:
'teeth' is the definition.
Although both the answer and definition are verbs in their base form, I don't understand how they can define each other.
'peg's short' is the wordplay.
'peg's' becomes 'tee's' (tee is a kind of peg).
'short' means to remove the last letter.
'tees' with its final letter removed is 'TEE'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for tee that I've seen before include "Mark", "Where a hole starts", "Start of a hole in golf", "Starting peg", "'Drive off it, half-laughing (3)'".)