Shady assassins essentially promising to eliminate leader (10)
I believe the answer is:
suspicious
'shady' is the definition.
(synonyms)
'assassins essentially promising to eliminate leader' is the wordplay.
'essentially' means to look at the middle letters.
'promising' becomes 'auspicious' (similar in meaning).
'to eliminate leader' means to remove the first letter.
The middle of 'assassins' is 's'.
'auspicious' with its initial letter removed is 'uspicious'.
's'+'uspicious'='SUSPICIOUS'
(Other definitions for suspicious that I've seen before include "Doubtful" , "open to doubt" , "Dodgy" , "questionable" , "Shady; untrusting" .)