Put in a metal box (3)
I believe the answer is:
can
'box' is the definition.
(thesaurus)
'put in a metal' is the wordplay.
I cannot quite understand how this works, but
'a' could be 'an' and 'an' is found within the answer.
A single letter 'c' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for can that I've seen before include "John" , "toilet" , "(Tin) container" , "Is able; container" , "let go in the US" .)