Cheat regularly on capsized mariner (4)
I believe the answer is:
noah
'mariner' is the definition.
(from bible: Noah's Ark)
'cheat regularly on capsized' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'capsized' says the letters should be written backwards.
The alternate letters of 'cheat' are 'ha'.
'ha'+'on'='haon'
'haon' back-to-front is 'NOAH'.
(Other definitions for noah that I've seen before include "Floater" , "Old ship builder" , "Biblical ark builder" , "Biblical skipper during the Flood" , "Patriarch of the Ark" .)