Cheat compiles horse dossier, covering horse’s heart (8)

'cheat' is the definition.
(I know that finagler is a type of cheat)

'dossier covering horse's heart' is the wordplay.
'dossier' becomes 'file' (I've seen this before).
'covering' indicates putting letters inside.
'horse' becomes 'nag' (term for an old horse).
'heart' becomes 'r' (I can't justify this - if you can you should give a lot more credence to this answer).
'file' placed around 'nag' is 'finagle'.
'finagle'+'r'='FINAGLER'

'compiles horse' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.

Can you help me to learn more?