It takes a crowd outside to cause trouble (7)
I believe the answer is:
agitate
'to cause trouble' is the definition.
(I've seen this before)
'it takes a crowd outside' is the wordplay.
'takes' means one lot of letters goes inside another (take can mean to capture or invade).
'crowd outside' becomes 'gate' (gate can refer to the crowd for a sporting event. I am not sure about the 'outside' bit.).
'a'+'gate'='agate'
'it' put inside 'agate' is 'AGITATE'.
Can you help me to learn more?
(Other definitions for agitate that I've seen before include "Fluster" , "Make nervous; stir" , "Unsettle" , "Disturb, cause to worry" , "Disturb or shake" .)