Most formal mega-performer, I boxed (10)

'most formal' is the definition.
('starchy' can be similar in meaning to 'formal')

'mega-performer i boxed' is the wordplay.
I cannot really see how this works, but
'performer' could be 'star' (star is a kind of performer) and 'star' is found in the answer.
'i' is present in the answer.
The remaining letters 'chest' is a valid word 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?