After knock, taps turn each way (4,3,3,4)
I believe the answer is:
blow hot and cold
'turn each way' is the definition.
(behave in different ways)
'after knock taps' is the wordplay.
'after' is a charade indicator (letters next to each other).
'knock' becomes 'blow' (both can mean a hit).
'taps' becomes 'hot and cold' (hot and cold taps).
'blow'+'hotandcold' is 'BLOW HOT AND COLD'.
(Other definitions for blow hot and cold that I've seen before include "not steady" , "may waver" , "as will the sirocco and mistral?" , "Be unable to decide" , "behave inconsistently" .)