Drug gang's leader is ambitious (8)
I believe the answer is:
aspiring
'ambitious' is the definition.
(similar in meaning)
'drug gang's leader' is the wordplay.
'drug' becomes 'aspirin' (aspirin is a kind of drug).
'gang's leader' becomes 'g' (I can't justify this - if you can you should believe this answer much more).
'aspirin'+'g'='ASPIRING'
'is' is the link.
Can you help me to learn more?
(Other definitions for aspiring that I've seen before include "Having ambition to achieve greater things" , "Having a strong desire to achieve something" , "Hoping" , "Striving" , "Strongly desiring to achieve" .)