Notice spinners turning (4)
I believe the answer is:
spot
'notice' is the definition.
(spot can mean to notice or observe)
'spinners turning' is the wordplay.
'spinners' becomes 'tops' (things that spin).
'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 "moral flaw" , "See - blemish" , "See 12" , "Lady Macbeth: ''Out, damned . . . .!''" , "Small mark; observe" .)