Modify trailer fitting (5)
I believe the answer is:
adapt
'modify' is the definition.
(thesaurus)
'trailer fitting' is the wordplay.
'trailer' becomes 'ad' (I've seen this in other clues).
'fitting' becomes 'apt' (I've seen this before).
'ad'+'apt'='ADAPT'
(Other definitions for adapt that I've seen before include "Tailor" , "Convert to another use or situation" , "Adjust (to new conditions)" , "match" , "Accommodate" .)