Genuine regret following Tory leader (4)
I believe the answer is:
true
'genuine' is the definition.
('true' can be similar in meaning to 'genuine')
'regret following tory leader' is the wordplay.
'regret' becomes 'rue' ('rue' can be a synonym of 'regret').
'following' means one lot of letters go next to another.
'tory leader' becomes 't' (I've seen this in other clues).
'rue' put after 't' is 'TRUE'.
(Other definitions for true that I've seen before include "Exact; faithful" , "Veritable" , "Genuine, loyal" , "Genuine, real" , "A false alternative?" .)