French mark (7)
I believe the answer is:
cedilla
'french mark' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cedilla that I've seen before include "mark not seen in 18 [FACADE]?" , "Small mark placed below C in Portuguese and Catalan, for example" , "for French character" , "Sign to soften C" , "Diacritical mark" .)