Heads back in daze (4)
I believe the answer is:
stun
'daze' is the definition.
(I know that daze can be written as stun)
'heads back' is the wordplay.
'heads' becomes 'nuts' (I've seen this before).
'back' says the letters should be written in reverse.
'nuts' in reverse letter order is 'STUN'.
'in' is the link.
(Other definitions for stun that I've seen before include "Knock unconscious" , "Anaesthetise" , "Leave speechless" , "Make senseless or dizzy with a blow" , "Daze, paralyse" .)