Steal identity, an informal one (8)
I believe the answer is:
nickname
'an informal one' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'steal identity' is the wordplay.
'steal' becomes 'nick' (nick can informally mean to steal).
'identity' becomes 'name' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'nick'+'name'='NICKNAME'
Can you help me to learn more?
(Other definitions for nickname that I've seen before include "Familiar term of address" , "Jack the Ripper, for example" , "Add soubriquet to" , "Not the one one is christened with" , "Cognomen" .)