Fire very old Tory leader that's injected heroin (5)
I believe the answer is:
shoot
'fire' is the definition.
(shooting is a kind of firing)
'very old tory leader that's injected heroin' is the wordplay.
'very' becomes 'so' (both can mean 'extremely').
'leader' suggests taking the first letters.
'that's injected' is an insertion indicator.
'heroin' becomes 'H' (synonyms).
The initial letters of 'old tory' is 'ot'.
'so'+'ot'='soot'
'soot' enclosing 'h' is 'SHOOT'.
(Other definitions for shoot that I've seen before include "Say what you have to say" , "away" , "New growth; last word of Hamlet" , "Kill; part of plant" , "Use a gun; film" .)