An American can (12)
I believe the answer is:
penitentiary
'can' is the definition.
(thesaurus)
'an american' is the wordplay.
I cannot really understand how this works, but
'an' could be 'i' (Roman numeral for one) and 'i' is found in the answer.
'american' could be 'a' (common abbreviation - e.g. in organisation names) and 'a' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for penitentiary that I've seen before include "American prison" , "Senior Catholic" , "US prison" , "stir" .)