Firm presented with American coin of historical interest (7)
I believe the answer is:
solidus
'coin of historical interest' is the definition.
(solidus was a historical type of coin)
'firm presented with american' is the wordplay.
'firm' becomes 'solid' (synonyms).
'presented with' is a charade indicator (letters next to each other).
'american' becomes 'us' (abbreviation for United States).
'solid'+'us'='SOLIDUS'
(Other definitions for solidus that I've seen before include "old coin" , "Oblique stroke, slash" , "Slash symbol" .)