Very large centre in Toulouse city (4)
I believe the answer is:
oslo
'city' is the definition.
(capital of Norway)
'very large centre in toulouse' is the wordplay.
'very large' becomes 'os' (abbreviation for oversize).
'centre in' indicates the central letters.
The middle letters of 'toulouse' are 'lo'.
'os'+'lo'='OSLO'
(Other definitions for oslo that I've seen before include "foreign port" , "European capital, known until 1924 as Christiania" , "Nordic capital" , "Go solo to European capital" , "Norwegian chief port" .)