Leader of Russian fliers feels remorse (7)
I believe the answer is:
regrets
'feels remorse' is the definition.
(I've seen this in another clue)
'leader of russian fliers' is the wordplay.
'leader of russian' becomes 'r' (I can't explain this - if you can you should believe this answer much more).
'fliers' becomes 'egrets' (egret is a kind of bird).
'r'+'egrets'='REGRETS'
Can you help me to learn more?
(Other definitions for regrets that I've seen before include "Rues" , "Bemoans" , "Sorrows" .)