Birds initially ignored nuts (3)
I believe the answer is:
ens
'nuts' is the definition.
(in printing nut can mean a space as wide as an 'n')
'birds initially ignored' is the wordplay.
'birds' becomes 'pens' (pen is a female swan).
'initially ignored' means to remove the first letter.
'pens' with its first letter taken off is 'ENS'.
(Other definitions for ens that I've seen before include "Printing measures" , "Entity" .)