Regret adopting son's cunning plan (4)
I believe the answer is:
ruse
'cunning plan' is the definition.
(I've seen this before)
'regret adopting son's' is the wordplay.
'regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'adopting' is an insertion indicator.
'rue' enclosing 's' is 'RUSE'.
(Other definitions for ruse that I've seen before include "Sue to be a ploy" , "Cunning scheme" , "Trick, stratagem" , "trick played" , "Sure to be a trick" .)