Knock Pat over (3)
I believe the answer is:
tap
'knock' is the definition.
(synonyms)
'pat over' is the wordplay.
'over' is a reversal indicator.
'pat' in reverse letter order is 'TAP'.
(Other definitions for tap that I've seen before include "Hit; bug" , "Light touch (hot or cold?)" , "indicate availability" , "Somewhere to drink beer" , "Sort of dance" .)