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