Teheran vacillating after Tory leader's menace (8)
I believe the answer is:
threaten
'menace' is the definition.
('threaten' can be a synonym of 'menace')
'teheran vacillating after tory leader's' is the wordplay.
'vacillating' indicates anagramming the letters.
'after' means one lot of letters go next to another.
'tory leader' becomes 't' (I've seen this in other clues).
'teheran' with letters rearranged gives 'hreaten'.
'hreaten' after 't' is 'THREATEN'.
(Other definitions for threaten that I've seen before include "Utter menaces" , "Give warning of something unpleasant" , "Look likely to" , "Impend, portend" , "loom" .)