Encountered a trainee copper, say (5)
I believe the answer is:
metal
'copper say' is the definition.
(copper is a kind of metal)
'encountered a trainee' is the wordplay.
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'trainee' becomes 'l' (as in an L-plate for a learner driver).
'met'+'a'+'l'='METAL'
(Other definitions for metal that I've seen before include "Musically it can be heavy" , "Headbanger's music genre" , "Broken stone used in road-making" , "Eg, copper" , "Broken stones" .)