Whirling water concealed by breached dyke (4)
I believe the answer is:
eddy
'whirling water' is the definition.
(I've seen this in another clue)
'concealed by breached dyke' is the wordplay.
'concealed by' indicates the answer is hidden within the clue.
'EDDY' can be found hidden inside 'breached dyke'.
(Other definitions for eddy that I've seen before include "Vortex" , "Religious woman" , "current running back" , "Counter-current" , "She founded Christian Science in 1866, d. 1910" .)