Unconcerned, first street gang to gain power (9)
I believe the answer is:
impassive
'unconcerned' is the definition.
(I've seen this before)
'first street gang to gain power' is the wordplay.
'first' becomes 'i' (I in Roman numerals eg Elizabeth I).
'street gang' becomes 'massive' (I can't justify this - if you can you should believe this answer much more).
'to gain' is an insertion indicator.
'power' becomes 'p'.
'massive' going around 'p' is 'mpassive'.
'i'+'mpassive'='IMPASSIVE'
Can you help me to learn more?
(Other definitions for impassive that I've seen before include "Not showing emotions in face" , "Inscrutable, not showing emotion" , "poker-faced" , "Expressionless" , "Undemonstrative" .)