It takes a crowd gathered outside to cause trouble (7)
I believe the answer is:
agitate
'cause trouble' is the definition.
(I've seen this before)
'it takes a crowd gathered outside' is the wordplay.
'takes' means one lot of letters goes inside another (take can mean to capture or invade).
'crowd gathered outside' becomes 'gate' (gate can mean the crowd for a sporting event).
'a'+'gate'='agate'
'it' placed into 'agate' is 'AGITATE'.
'to' acts as a link.
(Other definitions for agitate that I've seen before include "Fluster" , "Upset, disturb the surface of" , "rock" , "cause trouble" , "'Stir, excite (7)'" .)