Most traditional plaza at the top of street (8)

'most traditional' is the definition.
(square can mean conventional or old-fashioned)

'plaza at the top of street' is the wordplay.
'plaza' becomes 'square' (synonyms).
'at' means one lot of letters go next to another.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'top of' indicates taking the first letters.
The initial letter of 'street' is 's'.
't' put after 's' is 'st'.
'square'+'st'='SQUAREST'