A resource since fixed (5)
I believe the answer is:
asset
'a resource' is the definition.
(I've seen this in another clue)
'since fixed' is the wordplay.
'since' becomes 'as' (I've seen this in other clues).
'fixed' becomes 'set' (both can mean decided or determined).
'as'+'set'='ASSET'
(Other definitions for asset that I've seen before include "Something valuable to have" , "Item well worth having" , "Something useful" , "valuable sort" , "one liable to be stripped?" .)