Grit regularly swallowed by cardinal bird (6)
I believe the answer is:
martin
'bird' is the definition.
(martin is a kind of bird)
'grit regularly swallowed by cardinal' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'swallowed by' means one lot of letters goes inside another.
'cardinal' becomes 'main' (I've seen this in other clues).
The alternating letters of 'grit' are 'rt'.
'rt' placed within 'main' is 'MARTIN'.
(Other definitions for martin that I've seen before include "Tweeter" , "25 across [CRANE]" , "Variety of swallow" , "Dickens novel partly set in America" , "Bird; in tram (anag.)" .)