Examination of gold and aluminium (4)
I believe the answer is:
oral
'examination' is the definition.
(oral is a kind of examination)
'gold and aluminium' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'and' means one lot of letters go next to another.
'aluminium' becomes 'al' (Al is the chemical symbol for aluminium).
'or'+'al'='ORAL'
'of' acts as a link.
(Other definitions for oral that I've seen before include "Spoken rather than written" , "Via the mouth" , "Of the voice" , "See 18" , "Ant; tourist (dial.)" .)