Part of the body to slap (4)
I believe the answer is:
knee
'part of the body to slap' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for knee that I've seen before include "Capped joint?" , "Genuflector?" , "Major joint in leg" , "may present problem to maid" , "Hinged joint" .)