Regretting downfall: little good (5)
I believe the answer is:
ruing
'regretting' is the definition.
('rue' can be a synonym of 'regret')
'downfall little good' is the wordplay.
'downfall' becomes 'ruin' (I've seen this before).
'little' suggests taking the first letters.
The first letter of 'good' is 'g'.
'ruin'+'g'='RUING'
(Other definitions for ruing that I've seen before include "Regretful" , "Feeling sorry for" , "Bitterly regretting" , "Feeling bad" , "Having second thoughts" .)