Drive truck back surrounded by monkeys (7)
I believe the answer is:
impetus
'drive' is the definition.
(impetus is a drive to do something)
'truck back surrounded by monkeys' is the wordplay.
'truck' becomes 'ute' (Australian abbreviation for utility vehicle).
'back' says the letters should be written backwards.
'surrounded by' means one lot of letters goes inside another.
'monkeys' becomes 'imps' (both can mean mischievous children).
'ute' written backwards gives 'etu'.
'etu' placed into 'imps' is 'IMPETUS'.
(Other definitions for impetus that I've seen before include "Imputes to force with which a body moves" , "Incentive - time's up (anag)" , "influence" , "Driving force, momentum" , "Catalyst" .)