Tory leader isn't commonly for smear (5)
I believe the answer is:
taint
'smear' is the definition.
(thesaurus)
'tory leader isn't commonly' is the wordplay.
'tory leader' becomes 't' (I've seen this in other clues).
'isn't commonly' becomes 'aint' (I've seen this in other clues).
't'+'aint'='TAINT'
'for' is the link.
(Other definitions for taint that I've seen before include "black mark" , "Defile" , "It isn't commonly" , "Sully, stain" , "Corrupt" .)