Jon, rubbish actor first seen in Mad Men (4)

'jon rubbish actor' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.

'first seen in mad men' is the wordplay.
'first' becomes 'a' (first letter of the alphabet).
'seen in' is an insertion indicator.
'mad' becomes 'h' (I can't explain this - if you can you should believe this answer much more).
'men' becomes 'mm' (N is short for knight - man can mean a chess piece).
'h'+'mm'='hmm'
'a' going within 'hmm' is 'HAMM'.

Can you help me to learn more?