Toy bear losing head in whirlpool (4)
I believe the answer is:
eddy
'whirlpool' is the definition.
(an eddy is a small whirlpool)
'toy bear losing head' is the wordplay.
'toy bear' becomes 'teddy' (teddy bear).
'losing head' means to remove the first letter.
'teddy' with its initial letter taken off is 'EDDY'.
'in' is the link.
(Other definitions for eddy that I've seen before include "Current running against the main stream, causes small whirlpool" , "Founder of Christian Science, d. 1910" , "Ripple" , "Chap" , "moving water" .)