Whirl concealed by breached dyke (4)
I believe the answer is:
eddy
'whirl' is the definition.
(synonyms)
'concealed by breached dyke' is the wordplay.
'concealed by' says the answer is hidden in the clue.
'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" .)