Gold, aluminium in the mouth (4)
I believe the answer is:
oral
'in the mouth' is the definition.
(I've seen this in another clue)
'gold aluminium' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'aluminium' becomes 'al' (Al is the chemical symbol for aluminium).
'or'+'al'='ORAL'
(Other definitions for oral that I've seen before include "Unwritten examination" , "language exam" , "Via the mouth" , "Using speech rather than writing" , "Exam using speech" .)