Rap Kelvin and knock over (4)
I believe the answer is:
knap
'rap' is the definition.
('knap' can be a synonym of 'rap')
'kelvin and knock over' is the wordplay.
'kelvin' becomes 'k' ('K' can be a synonym of 'kelvin').
'and' means one lot of letters go next to another.
'knock' becomes 'pan' (both can mean to criticise).
'over' says the letters should be written backwards.
'pan' in reverse letter order is 'nap'.
'k'+'nap'='KNAP'
(Other definitions for knap that I've seen before include "Rap" , "Strike stone" , "Deal hammer-blow" , "Shape (a flint) by striking" , "Break" .)