Whirlwind characterised dynamo in part (4)
I believe the answer is:
eddy
'whirlwind' is the definition.
(I've seen this before)
'characterised dynamo in part' is the wordplay.
'in part' indicates the answer is hidden within the clue.
'EDDY' can be found hidden inside 'characterised dynamo'.
(Other definitions for eddy that I've seen before include "A small swirl in water" , "religious leader" , "Small circular movement of water" , "Spin" , "Spinning current" .)