Historic city mostly held in regard, looking back (10)
I believe the answer is:
retrospect
'looking back' is the definition.
(I've seen this before)
'historic city mostly held in regard' is the wordplay.
'historic city' becomes 'troy' (ancient city).
'mostly' means to remove the last letter.
'held in' means one lot of letters goes inside another.
'regard' becomes 'respect' (respecting is a kind of regarding).
'troy' with its last letter removed is 'tro'.
'tro' inserted into 'respect' is 'RETROSPECT'.
(Other definitions for retrospect that I've seen before include "Look at history" , "Look(ing) back" , "A backward view" , "review" , "Hindsight" .)