Regularly dissolve something in water (4)
I believe the answer is:
isle
'something in water' is the definition.
(piece of land located in water)
'regularly dissolve' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'dissolve' are 'ISLE'.
(Other definitions for isle that I've seen before include "Wight or Man, say" , "''There is an ..., a bonny ...'' (anthem of Shannon RFC)" , "Of Man or Youth, for example" , "Land encircled by water" , "description of Britain" .)