Join after Tory mistake (7)
I believe the answer is:
confuse
'mistake' is the definition.
(confusing is a kind of mistaking)
'join after tory' is the wordplay.
'join' becomes 'fuse'.
'after' is a charade indicator (letters next to each other).
'tory' becomes 'con' (abbreviation for Conservative).
'fuse' put after 'con' is 'CONFUSE'.
(Other definitions for confuse that I've seen before include "Disconcert, befuddle" , "Muddle, mix up" , "Perplex" , "Bewilder" , "Puzzle" .)