Opening gold reserve holding one key (7)
I believe the answer is:
orifice
'opening' is the definition.
(I know that opening can be written as orifice)
'gold reserve holding one key' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'reserve' becomes 'ice' (both can mean aloofness).
'holding' means one lot of letters goes inside another.
'one' becomes 'I' (Roman numeral).
'key' becomes 'f' (musical key).
'or'+'ice'='orice'
'i'+'f'='if'
'orice' placed around 'if' is 'ORIFICE'.
(Other definitions for orifice that I've seen before include "For CIE, I look for an opening" , "Space" , "Bodily opening, mouth of cavity" , "Mouth-like opening" , "Aperture" .)