Keep lock on fine gold (8)
I believe the answer is:
fortress
'keep' is the definition.
(thesaurus)
'lock on fine gold' is the wordplay.
'lock' becomes 'tress' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'fine' becomes 'f' (abbreviation).
'gold' becomes 'or' ('or' is the heraldic name for gold).
'f'+'or'='for'
'tress' put after 'for' is 'FORTRESS'.
(Other definitions for fortress that I've seen before include "Perhaps keep" , "Military stronghold" , "Place that's secure" , "Fortified defensive structure" , "Secure building" .)