Quick! Run away before trouble starts (5)
I believe the answer is:
fleet
'quick' is the definition.
(I've seen this before)
'run away before trouble starts' is the wordplay.
'run away' becomes 'flee' (I've seen this before**).
'before' means one lot of letters go next to another.
'starts' suggests taking the first letters.
The first letter of 'trouble' is 't'.
'flee'+'t'='FLEET'
(Other definitions for fleet that I've seen before include "Naval force or collection of buses" , "cars en masse?" , "Ships - quick!" , "vehicles owned by a company" , "Fast kind of armada?" .)