Marsh in fine country, not good (7)
I believe the answer is:
fenland
'marsh' is the definition.
(I know that marsh can be written as fenland)
'in fine country not good' is the wordplay.
'in' means one lot of letters goes inside another.
'fine' becomes 'f' (abbreviation e.g. condition of old coins, books).
'country' becomes 'land' (synonyms).
'not good' becomes 'en' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'f'+'land'='fland'
'fland' enclosing 'en' is 'FENLAND'.
Can you help me to learn more?
(Another definition for fenland that I've seen is " Marshy area".)