Inside man (8)
I believe the answer is:
prisoner
'inside man' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'inside man' is the wordplay.
'inside' becomes 'prison' (being 'inside' is being in prison).
'man' becomes 'er' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'prison'+'er'='PRISONER'
Can you help me to learn more?
(Other definitions for prisoner that I've seen before include "Person behind bars" , "Incarcerated person" , "One in gaol" , "Person held captive as by law" , "Senior PR (anag) - one in detention" .)