Whirl concealed by breached dyke (4)
I believe the answer is:
eddy
'whirl' is the definition.
(I know that whirl can be written as eddy)
'concealed by breached dyke' is the wordplay.
'concealed by' indicates a hidden word.
'EDDY' is hidden in the letters of 'breached dyke'.
(Other definitions for eddy that I've seen before include "Whirlwind" , "Water circulating" , "Christian Scientist" , "Twist" , "Little swirl in water" .)