An affair can become involved in a capital city (7)
I believe the answer is:
romance
'an affair' is the definition.
(I've seen this in another clue)
'can become involved in a capital city' is the wordplay.
'become' indicates an anagram.
'involved in' is an insertion indicator.
'a capital city' becomes 'rome' (I've seen this before).
'can' with letters rearranged gives 'anc'.
'anc' put into 'rome' is 'ROMANCE'.
(Other definitions for romance that I've seen before include "Novel - love story" , "mysterious quality" , "Language group; love affair" , "Lie" , "Love-story" .)