Sacred bird, one seen by Brits regularly (4)
I believe the answer is:
ibis
'sacred bird' is the definition.
(bird considered sacred in ancient Egypt)
'one seen by brits regularly' is the wordplay.
'one' becomes 'i' (Roman numeral).
'seen by' means one lot of letters go next to another.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'brits' are 'bis'.
'i'+'bis'='IBIS'
(Other definitions for ibis that I've seen before include "Eastern" , "Sacred Egyptian bird" , "Hotel group" , "One has large bill" , "Revered bird" .)