Good on imposing French city! (8)
I believe the answer is:
grenoble
'french city' is the definition.
(I've seen this before)
'good on imposing' is the wordplay.
'good' becomes 'g' (abbreviation).
'on' becomes 're' ('on' can be mean 'with reference to').
'imposing' becomes 'noble' ('noble' can be a synonym of 'imposing').
'g'+'re'+'noble'='GRENOBLE'
(Other definitions for grenoble that I've seen before include "European city" , "City of southeast France" , "City of SE France" , "City of south-eastern France" , "French city and skiing centre" .)