Trespasser gendarme traps climbing exit (6)
I believe the answer is:
egress
'exit' is the definition.
(thesaurus)
'trespasser gendarme traps climbing' is the wordplay.
'traps' indicates the answer is hidden within the clue (the answer is 'trapped' somewhere in the clue).
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'sserge' is hidden within 'trespasser gendarme'.
'sserge' in reverse letter order is 'EGRESS'.
(Other definitions for egress that I've seen before include "Issue" , "Escape route" , "Emergence, coming out" , "Emergence - exit" , "Way out or means of exit" .)