Unpleasant consequences could be dead nasty, summit agreed (5,2,3)

'unpleasant consequences could' is the definition.
I know nothing about this answer so I can't judge whether this works.

'dead nasty summit agreed' is the wordplay.
I cannot really see how this works, but
'dead' could be 'd' (abbreviation in government records) and 'd' is found within the answer.
'summit' could be 'top' (synonyms) and 'top' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

'be' acts as a link.

Can you help me to learn more?