Test gold, aluminium (4)
I believe the answer is:
oral
'test' is the definition.
(I've seen this before)
'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 "By word of mouth, like some exams" , "Unwritten examination" , "Vocal test" , "Relating to speech" , "See 18" .)