A lot of hard work to secure answer for city (7)
I believe the answer is:
Here is my best explanation:
'city' is the definition.
(Swansea is an example)
'a lot of hard work to secure answer' is the subsidiary indication.
'a lot of' means to remove the last letter.
'hard work' becomes 'sweat' ('sweat' can be a synonym of 'hard work').
'to secure' is an insertion indicator.
'answer' becomes 'ans' (A common abbreviation).
'sweat' with its last letter removed is 'swea'.
'swea' placed around 'ans' is 'swansea'.
'for' is the link.
(Other definitions for swansea that I've seen before include "Second city of Wales", "Birthplace of Dylan Thomas", "City of South Wales", "Port of South Wales", "Welsh city", "Welsh port", "coastal location", "City of south Wales", "Home of the DVLA" and "Ecstasy, a win! (anag)".)