Possible Clues:

• Clear that summer's arrived
• 'Bound to be the start of summer then (6,4)'
• Then ''Summer is icumen in'' - clearly bound to be
• Bound to mean ''Summer is icumen in''
• Bound to be the beginning of summer afterwards
• Bound to be the beginning of summer
• That's bound to mean that summer is there
• When it's this, it's bound to be summer
• It's bound to make it clear that its summer now
• Bound to get to the beginning of summer, that's clear