Collusion to study transport in French city (10)
I believe the answer is:
connivance
'collusion' is the definition.
('connivance' can be a synonym of 'collusion')
'study transport in french city' is the wordplay.
'study' becomes 'con' (I've seen this before).
'transport' becomes 'van' (van is a kind of transport**).
'in' means one lot of letters goes inside another.
'french city' becomes 'nice'.
'van' going into 'nice' is 'nivance'.
'con'+'nivance'='CONNIVANCE'
'to' acts as a link.
(Other definitions for connivance that I've seen before include "working in cahoots" , "Passive co-operation" , "Act of conspiracy" .)