Supply long, trailing cape (5)
I believe the answer is:
cache
'supply' is the definition.
(thesaurus)
'long trailing cape' is the wordplay.
'long' becomes 'ache' ('ache' can be a synonym of 'long'**).
'trailing' means one lot of letters go next to another.
'cape' becomes 'c' (abbreviation used on maps).
'ache' put after 'c' is 'CACHE'.
(Other definitions for cache that I've seen before include "Collection of hidden things" , "Put away" , "Hiding place for treasure or store" , "Hidden storage space" , "Something hidden" .)