Fear Tory leader's mistake (6)
I believe the answer is:
terror
'fear' is the definition.
(terror is a kind of fear)
'tory leader's mistake' is the wordplay.
'tory leader' becomes 't' (I've seen this in other clues).
'mistake' becomes 'error' ('error' can be a synonym of 'mistake').
't'+'error'='TERROR'
(Other definitions for terror that I've seen before include "(What causes) panic" , "Mischief-maker; dread" , "consternation" , "Troublesome child" , "rogue" .)