Walk miles to be caught in ambush (5)
I believe the answer is:
tramp
'walk' is the definition.
(tramp can mean to walk around)
'miles to be caught in ambush' is the wordplay.
'miles' becomes 'm' (common abbreviation).
'to be caught in' means one lot of letters goes inside another.
'ambush' becomes 'trap' (synonyms).
'm' put within 'trap' is 'TRAMP'.
(Other definitions for tramp that I've seen before include "Merchant ship" , "sometimes harboured?" , "Vagrant, long walk" , "Bum!" , "Vagrant, drifter" .)