Men gatecrashing trendy parties within house (7)
I believe the answer is:
indoors
'within house' is the definition.
(I've seen this before)
'men gatecrashing trendy parties' is the wordplay.
'men' becomes 'OR' (Other Ranks: non-commissioned soldiers).
'gatecrashing' means one lot of letters goes inside another.
'trendy' becomes 'in' ('in' can mean fashionable).
'parties' becomes 'dos' (do is a kind of party).
'in'+'dos'='indos'
'or' going into 'indos' is 'INDOORS'.
(Other definitions for indoors that I've seen before include "Having returned to pavilion" , "home" , "Not outside" , "Covered" , "With a roof over one's head?" .)