River bird to feel remorse (6)
I believe the answer is:
regret
'feel remorse' is the definition.
(I've seen this in another clue)
'river bird' is the wordplay.
I cannot really see how this works, but
'river' could be 'r' (abbreviation for river) and 'r' is present in the answer.
'bird' could be 'egret' (egret is a kind of bird) and 'egret' is located in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'to' is the link.
Can you help me to learn more?
(Other definitions for regret that I've seen before include "Feel sorry about" , "Feel remorse for" , "Be sorry for, wish it hadn't happened" , "Be sorry for an action" , "Deprecate" .)