I'm in the French capital (4)
I believe the answer is:
lima
'capital' is the definition.
(Lima is an example)
'i'm in the french' is the wordplay.
'in' means one lot of letters goes inside another.
'the french' becomes 'la' (**).
'im' inserted inside 'la' is 'LIMA'.
(Other definitions for lima that I've seen before include "Edible bean; capital city" , "city (like 22)" , "Mail to capital of Peru" , "distant city" , "Mail to Peru'd capital" .)