Like, for example, a test of metal (5)
I believe the answer is:
assay
'a test of metal' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'like for example' is the wordplay.
'like' becomes 'as'.
'for example' becomes 'say' ('say' can be used to introduce an example).
'as'+'say'='ASSAY'
Can you help me to learn more?
(Other definitions for assay that I've seen before include "Test metal for quality" , "Test, check" , "'Analyse, precious metal, say (5)'" , "Test gold" , "Test, or say, to establish quality" .)