Bird that's nesting in bush again (4)
I believe the answer is:
shag
'bird' is the definition.
(I've seen this before)
'nesting in bush again' is the wordplay.
'nesting in' says the answer is hidden in the clue.
'SHAG' is hidden within 'bush again'.
'that's' acts as a link.
(Other definitions for shag that I've seen before include "Coarse tobacco - cormorant" , "Diver" , "flighty creature" , "Green cormorant" , "Bird - dance" .)