The French kiss is sloppy (3)
I believe the answer is:
'sloppy' is the definition.
(both can careless)
'the french kiss' is the wordplay.
'the french' becomes 'la' ('the' in French).
'kiss' becomes 'x' (X can represent a kiss).
'is' is the link.
(Other definitions for lax that I've seen before include "Casual" , "Flaccid" , "Not taut" , "Not strict or conscientious" , "Insufficiently strict" .)