Knock off laps (4)
I believe the answer is:
slap
'knock' is the definition.
(thesaurus)
'off laps' is the wordplay.
'off' indicates anagramming the letters.
'laps' anagrammed gives 'SLAP'.
(Other definitions for slap that I've seen before include "Make-up (slang)" , "Mild punishment" , "Strike action" , "Hit with an open hand" , "Blow (in face, on back, wrist)" .)