Pop peg back (3)
I believe the answer is:
nip
'pop' is the definition.
('nip out' and 'pop out' can both mean to go out)
'peg back' is the wordplay.
'peg' becomes 'pin' ('pin' can be a synonym of 'peg').
'back' shows that the letters should be reversed in order.
'pin' backwards is 'NIP'.
(Other definitions for nip that I've seen before include "small draught" , "small amount to drink" , "Feeling cold" , "In the air, it feels cold" , "Pinch, maybe in the bud" .)