Clay found in marl pit regularly (3)
I believe the answer is:
ali
'clay' is the definition.
(Muhammad Ali was born Cassius Clay)
'marl pit regularly' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'marl'+'pit'='marlpit'
The alternate letters of 'marlpit' are 'ALI'.
'found in' acts as a link.
(Other definitions for ali that I've seen before include "Muhammad ___ (or MacGraw?)" , "Mohammad . . ., . . . Hewson" , "Muhammad or Baba?" , "Fourth caliph of Islam" , "Champion of ring" .)