Tag prison's given to backsliding English chap (8)
I believe the answer is:
nickname
'tag' is the definition.
(I've seen this before)
'prison's given to backsliding english chap' is the wordplay.
'prison' becomes 'nick' (slang term for prison).
'given to' is a charade indicator (letters next to each other).
'backsliding' is a reversal indicator.
'english chap' becomes 'eman' (I can't explain this - if you can you should give a lot more credence to this answer).
'eman' backwards is 'name'.
'nick'+'name'='NICKNAME'
Can you help me to learn more?
(Other definitions for nickname that I've seen before include "Affectionate or invented appellation, not one's own" , "Friendly appellation by which person is known" , "Familiar moniker" , "tag" , "Jack the Ripper, for example" .)