The French mark continental leader with allied slur (7)
I believe the answer is:
cedilla
'the french mark' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for cedilla that I've seen before include "for French character" , "The mark under the letter C in Spanish" , "Mark, in foreign language" , "Mark made under a 'c'" , "foreign accent" .)