Gold that is pounds light (5)
I believe the answer is:
oriel
'light' is the definition.
(thesaurus)
'gold that is pounds' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'pounds' becomes 'l' (abbreviation e.g. Lsd - pounds, shillings, pence).
'or'+'ie'+'l'='ORIEL'
(Other definitions for oriel that I've seen before include "Style of window" , "Upper-story bay window" , "Protruding window" , "polygonal recess" , "lecturer seen near Lorraine regularly" .)