Spin the headless donkey (4)
I believe the answer is:
eddy
'spin' is the definition.
(I've seen this before)
'the headless donkey' is the wordplay.
'the headless' means to remove the first letter.
'donkey' becomes 'neddy' (I've seen this before).
'neddy' with its first letter taken away is 'EDDY'.
(Other definitions for eddy that I've seen before include "Spinning current" , "making the rounds?" , "Vortex" , "move in circles" , "Circular movement of air" .)