Ambitious poisoner managed to inject it (8)
I believe the answer is:
aspirant
'ambitious' is the definition.
(similar in meaning)
'poisoner managed to inject it' is the wordplay.
'poisoner' becomes 'asp' (venomous snake).
'managed' becomes 'ran' (I've seen this in another clue).
'to inject' means one lot of letters goes inside another (inserted letters are injected).
'ran' placed inside 'it' is 'irant'.
'asp'+'irant'='ASPIRANT'
(Other definitions for aspirant that I've seen before include "expressing high ambition?" , "Partisan sort of hopeful" , "Partisan (anag)" , "Candidate; one with a desire to succeed" , "Hopeful one" .)