Keen knight in hammered metal (6)
I believe the answer is:
lament
'keen' is the definition.
(keen can mean to wail with grief)
'knight in hammered metal' is the wordplay.
'knight' becomes 'n' (chess abbreviation).
'in' is an insertion indicator.
'hammered' indicates anagramming the letters (slang term for drunk).
'metal' anagrammed gives 'lamet'.
'n' inserted within 'lamet' is 'LAMENT'.
(Other definitions for lament that I've seen before include "Mourn (a person's death)" , "Mourn for; deplore" , "Song expressing grief" , "Express disappointment" , "Cry of sorrow" .)