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