Regrets a stratagem, by the sound of it (4)
I believe the answer is:
rues
'regrets' is the definition.
('rue' can be a synonym of 'regret')
'a stratagem by the sound of it' is the wordplay.
'a stratagem' becomes 'ruse' (a ruse is a stratagem or ploy).
'by the sound of it' shows a homophone (sound like).
'ruse' is a homophone of 'RUES'.
(Other definitions for rues that I've seen before include "Is sorry for - plants" , "Regrets, to be sure" , "Bemoans" .)