Other fella seen regularly (4)
I believe the answer is:
else
'other' is the definition.
('else' can be similar in meaning to 'other')
'fella seen regularly' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'fella'+'seen'='fellaseen'
The alternating letters of 'fellaseen' are 'ELSE'.
(Other definitions for else that I've seen before include "Otherwise; different from" , "Besides; instead" , "Adverb meaning if not, otherwise" , "Eels (anag)" , "Besides, otherwise" .)