Doctor's enemy edging in trailer (3,6)

'doctor's enemy' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps you can see a link between them that I don't see?

'edging in trailer' is the wordplay.
'edging' becomes 'hem' (I've seen this in another clue).
'in' is an insertion indicator.
'trailer' becomes 'taster' (I can't explain this - if you can you should give a lot more credence to this answer).
'hem' going into 'taster' is 'THE MASTER'.

Can you help me to learn more?