Quick - run away before the trouble starts (5)
I believe the answer is:
fleet
'quick' is the definition.
(I've seen this before)
'run away before the trouble starts' is the wordplay.
'run away' becomes 'flee' (to flee is to run away).
'before' is a charade indicator (letters next to each other).
'starts' says to take the initial letters.
The initial letter of 'trouble' is 't'.
'flee'+'t'='FLEET'
(Other definitions for fleet that I've seen before include "Swift like navy" , "Ships - quick!" , "Fast - naval ships" , "Swift; armada" , "good turn of speed" .)