Most tainted talk-back that's starting to save time (7)

'most tainted' is the definition.
Both the definition and answer are superlative adjectives. Maybe you can see an association between them that I can't see?

'talk-back that's starting to save time' is the wordplay.
'talk' becomes 'mag' (I can't explain this - if you can you should give a lot more credence to this answer).
'back' says the letters should be written in reverse.
'that's' becomes 'ie'.
'starting to' suggests taking the first letters.
The first letters of 'save time' is 'st'.
'mag' written backwards gives 'gam'.
'gam'+'ie'+'st'='GAMIEST'

Can you help me to learn more?