Joke that’s disgusting - beneath the French (5)
I believe the answer is:
laugh
'joke' is the definition.
(synonyms)
'disgusting beneath the french' is the wordplay.
'disgusting' becomes 'ugh'.
'beneath' is a charade indicator (letters next to each other) (in a down clue).
'the french' becomes 'la'.
'ugh' after 'la' is 'LAUGH'.
'that's' acts as a link.
(Other definitions for laugh that I've seen before include "Express joy or amusement" , "Express amusement audibly" , "a ridiculous thing" , "Show amusement or pleasure" , "Chuckle" .)