Regarding French city trust (4,2)
I believe the answer is:
rely on
'trust' is the definition.
(relying on is a kind of trusting)
'regarding french city' is the wordplay.
'regarding' becomes 're'.
'french city' becomes 'lyon' (I've seen this before).
're'+'lyon'='RELY ON'
(Other definitions for rely on that I've seen before include "Trust, be confident in" , "Trust in with confidence" , "Put trust in" .)