Rue missing intro for White Bird (5)
I believe the answer is:
egret
'white bird' is the definition.
(I've seen this before)
'rue missing intro' is the wordplay.
'rue' becomes 'regret' ('regret' can be a synonym of 'rue').
'missing intro' means to remove the first letter.
'regret' with its initial letter taken away is 'EGRET'.
'for' acts as a link.
(Other definitions for egret that I've seen before include "White-feathered wading bird" , "Mainly white heron" , "Water-bird, has plume" , "One wading" , "Bird eating fish" .)