Volunteers needed in case of shooting party (4)
I believe the answer is:
stag
'party' is the definition.
(I've seen this before)
'volunteers needed in case of shooting' is the wordplay.
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'needed in' means one lot of letters goes inside another.
'case of' suggests removing the centre (outsides of).
'shooting' with its centre removed is 'sg'.
'ta' going into 'sg' is 'STAG'.
(Other definitions for stag that I've seen before include "Men-only" , "Animal - for men only" , "Hart" , "Stockmarket operator gambling on a quick profit" , "Investor" .)