Join a party, but not gregarious (7)
I believe the answer is:
asocial
'not gregarious' is the definition.
(I've seen this before)
'join a party' is the wordplay.
I cannot quite see how this works, but
'a' is within the answer.
'party' could be 'social' (social is a kind of party) and 'social' is found within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'but' is the link.
(Other definitions for asocial that I've seen before include "without friends" , "not connected with community" , "cold" , "Inconsiderate to others" , "lacking friendly instincts" .)