Maintain returns to swell money for capital (7)
I believe the answer is:
yerevan
'capital' is the definition.
(capital of Armenia)
'maintain returns to swell money for' is the wordplay.
'maintain' becomes 'aver' (averring is a kind of maintaining).
'returns' says the letters should be written backwards.
'to swell' means one lot of letters goes inside another (swell' can mean 'fill').
'money for' becomes 'yen' (I can't explain this - if you can you should believe this answer much more).
'aver' reversed gives 'reva'.
'reva' put inside 'yen' is 'YEREVAN'.
Can you help me to learn more?
(Other definitions for yerevan that I've seen before include "Capital of Armenia" , "east European city" , "ancient capital" .)