Regrets stratagem, say (4)
I believe the answer is:
rues
'regrets' is the definition.
('rue' can be a synonym of 'regret')
'stratagem say' is the wordplay.
'stratagem' becomes 'ruse' (I've seen this in another clue).
'say' 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" .)