Encountered a trainee copper (5)
I believe the answer is:
metal
'copper' is the definition.
(I've seen this before)
'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 "Aluminium or copper for instance" , "Copper or silver, say" , "Intrinsic quality; stone for roads" , "Iron, gold or silver, for example" , "Lead, say" .)