Tory leader has pain in bum (4)
I believe the answer is:
tail
'bum' is the definition.
('tail' can be a synonym of 'bum')
'tory leader has pain' is the wordplay.
'tory leader' becomes 't' (I've seen this in other clues).
'has' means one lot of letters go next to another.
'pain' becomes 'ail' ('ail' can be a synonym of 'pain').
't'+'ail'='TAIL'
'in' is the link.
(Other definitions for tail that I've seen before include "Elongated posterior part of an animal" , "Follow closely - taper (off)" , "Feebler batsmen" , "Rump" , "that can be cast by a lizard" .)