Mark under the letter C in French (7)
I believe the answer is:
cedilla
'mark under the letter c in french' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for cedilla that I've seen before include "Mark used under a c" , "Small mark placed below C in Portuguese and Catalan, for example" , "mark not seen in 18 [FACADE]?" , "Mark suggesting a 'c' should be pronounced an 's'" , "Sign to soften C" .)