It may be shot or longer drink, holding a party (9)
I believe the answer is:
paintball
'it may be shot' is the definition.
(paintballs are shot from guns)
'longer drink holding a party' is the wordplay.
'longer drink' becomes 'pint' (a pint is a large drink).
'holding' is an insertion indicator.
'party' becomes 'ball' (ball is a kind of party).
'pint' placed around 'a' is 'paint'.
'paint'+'ball'='PAINTBALL'
'or' acts as a link.
(Other definitions for paintball that I've seen before include "simulated combat" , "Missile used in war games" , "messy game" , "war game" .)