Start of summer as break is most traditional (8)

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

'start of summer as break' is the wordplay.
'start of' says to take the initial letters.
'as' becomes 'qua' (I've seen this in other clues).
'break' becomes 'rest' (resting is a kind of breaking).
The first letter of 'summer' is 's'.
's'+'qua'+'rest'='SQUAREST'

'is' acts as a link.