Metal unknown – that’s the paradox (5)
I believe the answer is:
irony
'the paradox' is the definition.
(thesaurus)
'metal unknown' is the wordplay.
'metal' becomes 'iron' (iron is a metal).
'unknown' becomes 'y' (algebra).
'iron'+'y'='IRONY'
'that's' acts as a link.
(Other definitions for irony that I've seen before include "sarcastic wit" , "like a metallic element" , "The use of words which is the opposite of what they say" , "Might this be dramatic" , "(Words) inferring the opposite of what's actually said" .)