Doctor sure to have regrets (4)
I believe the answer is:
rues
'regrets' is the definition.
('rue' can be a synonym of 'regret')
'doctor sure' is the wordplay.
'doctor' indicates an anagram.
'sure' anagrammed gives 'RUES'.
'to have' acts as a link.
(Other definitions for rues that I've seen before include "Is sorry for - plants" , "Bemoans" , "Regrets, to be sure" .)