Speak after as test for metal (5)
I believe the answer is:
assay
'test for metal' is the definition.
(I've seen this before)
'speak after as' is the wordplay.
'speak' becomes 'say' (I've seen this in other clues).
'after' says to put letters next to each other.
'say' put after 'as' is 'ASSAY'.
(Other definitions for assay that I've seen before include "Examine chemicals" , "Test, check" , "Test of substance" , "Test for metal content" , "Check (a metal's) quality" .)