Seabirds or seabird coming west (4)
I believe the answer is:
auks
'seabirds' is the definition.
(auk is a kind of seabird)
'seabird coming west' is the wordplay.
'seabird' becomes 'skua' (type of seabird).
'coming west' is a reversal indicator (letters go right to left or 'east to west').
'skua' in reverse letter order is 'AUKS'.
'or' acts as a link.
(Other definitions for auks that I've seen before include "Sticky diving seabirds" , "Fliers" , "Birds (such as razorbills)" , "Stocky diving birds" , "Northern black and white diving birds" .)