Say, Man dissolves at regular intervals (4)
I believe the answer is:
isle
'say man' is the definition.
(Isle of Man is an example)
'dissolves at regular intervals' is the wordplay.
'at regular intervals' means one should take alternating letters.
The alternate letters of 'dissolves' are 'ISLE'.
(Other definitions for isle that I've seen before include "Detached territory" , "Hawaii, perhaps" , "what Donne implied he wasn't" , "description of Britain" , "Small mass of land in the sea" .)