Regretting (5)
I believe the answer is:
ruing
'regretting' is the definition.
('rue' can be a synonym of 'regret')
This is the entire clue.
(Other definitions for ruing that I've seen before include "Regretful" , "Bitterly regretting" , "Feeling sorry for" , "Having second thoughts" , "full of remorse?" .)