Most untidy place for eating that is on street (8)

I believe the answer is:

messiest

'most untidy' is the definition.
('messy' can be a synonym of 'untidy')

'place for eating that is on street' is the wordplay.
'place for eating' becomes 'mess' (I've seen this before).
'that is' becomes 'i.e.' (short for Latin 'id est', 'that is').
'on street' becomes 'st' (I've seen this in other clues).
'mess'+'ie'+'st'='MESSIEST'

I've seen this clue in The Telegraph.

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