Child’s toy losing head in circular movement (4)
I believe the answer is:
eddy
'circular movement' is the definition.
(circular movement of water)
'child's toy losing head' is the wordplay.
'child's toy' becomes 'teddy' (teddy bear).
'losing head' means to remove the first letter.
'teddy' with its initial letter taken away is 'EDDY'.
'in' is the link.
(Other definitions for eddy that I've seen before include "Chap" , "Christian Scientist" , "Boy" , "Small circular movement of water" , "A small swirl in water" .)