City not big enough to fill small municipal area (5)
I believe the answer is:
paris
'city' is the definition.
(I know that Paris is an national capital)
'not big enough to fill small municipal area' is the wordplay.
'not big enough to fill' means to remove the last letter.
'small municipal area' becomes 'parish' (parish can mean a municipal area or district).
'parish' with its final letter taken away is 'PARIS'.
(Other definitions for paris that I've seen before include "where he got plastered?" , "He abducted Helen of Troy" , "Legendary prince" , "French capital - Trojan prince" , "I spar (anag)" .)