Notice heads turning (4)
I believe the answer is:
spot
'notice' is the definition.
(spot can mean to notice or observe)
'heads turning' is the wordplay.
'heads' becomes 'tops'.
'turning' shows that the letters should be reversed in order.
'tops' in reverse letter order is 'SPOT'.
(Other definitions for spot that I've seen before include "precise location" , "Sight - site" , "20 [MARK]" , "clock" , "make out" .)