Getting dangerously full, feed on substitute mainly, refined and graded (14)

'getting dangerously' is the definition.
I can't judge whether this defines the answer.

'full feed on substitute mainly refined and graded' is the wordplay.
I cannot really see how this works, but
'full' could be 'super' (similar in meaning) and 'super' is found in the answer.
'on' could be 'at' (I've seen this in other clues) and 'at' is present in the answer.
'refined' could be 'u' (abbreviation for upper-class - opposite to non-U) and 'u' is present in the answer.
'graded' could be 'rated' ('rate' can be a synonym of 'grade') and 'rated' is found in the answer.
A single letter 's' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?