Copper, say, encountered a trainee (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'.
'met'+'a'+'l'='METAL'
(Other definitions for metal that I've seen before include "Iron, for instance" , "Gold or silver in heraldry" , "Tungsten, for example" , "Broken stones" , "Maybe lead" .)