It's not true he's pretending to be a gangster (9)
I believe the answer is:
falsehood
'it's not true he's pretending' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'be a gangster' is the wordplay.
'be a' becomes 'false' (I can't explain this - if you can you should give a lot more credence to this answer).
'gangster' becomes 'hood' (I've seen this in another clue).
'false'+'hood'='FALSEHOOD'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for falsehood that I've seen before include "Deception" , "Untruth" , "Pork pie" , "work of fiction" , "Fictional account" .)