Encountered a large copper, say (5)
I believe the answer is:
metal
'copper say' is the definition.
(I know that copper is a type of metal)
'encountered a large' is the wordplay.
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'large' becomes 'L' (eg in clothes sizes).
'met'+'a'+'l'='METAL'
(Other definitions for metal that I've seen before include "Maybe lead" , "Copper possibly" , "Aluminium or copper for instance" , "Lead or iron, perhaps" , "Conducting solid" .)