Obtained leave to start trouble (3)
I believe the answer is:
got
'obtained' is the definition.
('get' can be a synonym of 'obtain')
'leave to start trouble' is the wordplay.
'leave' becomes 'go'.
'to' says to put letters next to each other.
'start' suggests taking the first letters.
The initial letter of 'trouble' is 't'.
'go'+'t'='GOT'
(Other definitions for got that I've seen before include "came by" , "Landed" , "Acquired" , "Received; understood" , "Caught" .)