Current disturbance in which donkey loses head (4)
I believe the answer is:
eddy
'current disturbance' is the definition.
'donkey loses head' is the wordplay.
'donkey' becomes 'neddy' (I've seen this before).
'loses head' means to remove the first letter.
'neddy' with its initial letter taken off is 'EDDY'.
'in which' acts as a link.
(Other definitions for eddy that I've seen before include "Whirlwind" , "A small whirl of water" , "Whirl (in fluid)" , "Swirl, whirl" , "Swirl; Duane -, US guitarist" .)