Stupid, in gatecrashing a function (7)
I believe the answer is:
asinine
'stupid' is the definition.
(I've seen this in another clue)
'in gatecrashing a function' is the wordplay.
'gatecrashing' indicates putting letters inside.
'function' becomes 'sine' (mathematical function).
'a'+'sine'='asine'
'in' placed inside 'asine' is 'ASININE'.
(Other definitions for asinine that I've seen before include "simple" , "Foolish like donkey" , "Idiotic" , "Foolish, silly" , "Stubborn" .)