Regretting destruction by leader of Goths (5)
I believe the answer is:
ruing
'regretting' is the definition.
('rue' can be a synonym of 'regret')
'destruction by leader of goths' is the wordplay.
'destruction' becomes 'ruin' (ruin is a kind of destruction).
'by' is a charade indicator (letters next to each other).
'leader of' indicates taking the first letters.
The initial letter of 'goths' is 'g'.
'ruin'+'g'='RUING'
(Other definitions for ruing that I've seen before include "full of remorse?" , "Feeling bad" , "Feeling sorry for" , "Bitterly regretting" , "feeling remorse" .)