Sieves covering Welsh birds (6)
I believe the answer is:
swifts
'birds' is the definition.
(swift is a kind of bird)
'sieves covering welsh' is the wordplay.
'sieves' becomes 'sifts' ('sift' can be a synonym of 'sieve').
'covering' is an insertion indicator.
'welsh' becomes 'W' (abbreviation).
'sifts' placed around 'w' is 'SWIFTS'.
(Other definitions for swifts that I've seen before include "wingers" , "Swallow-like birds" , "Small fast birds" .)