Impressive American city in Maine (7)
I believe the answer is:
augusta
'in maine' is the definition.
('Augusta' is part of 'maine')
'impressive american' is the wordplay.
'impressive' becomes 'august' (I've seen this before).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'august'+'a'='AUGUSTA'
'city' is the link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for augusta that I've seen before include "22 [MAINE] 's capital" , "US city" , "Algernon's aunt (Importance of Being Earnest)" , "Graham Greene's travelling aunt" , "Lady Bracknell" .)