Be the right size for beach initiations and other bouts of exertion (4,3,6)
I believe the answer is:
fits and starts
'other bouts of exertion' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'be the right size for beach initiations' is the wordplay.
'be the right size' becomes 'fit' (synonyms).
'for' means one lot of letters go next to another.
'beach' becomes 'sands' (sands can mean a beach area).
'initiations' becomes 'tarts' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'fit'+'sands'+'tarts'='FITS AND STARTS'
'and' is the link.
Can you help me to learn more?
(Other definitions for fits and starts that I've seen before include "Repeated bursts of activity" , "Irregular bursts of activity" , "Spasmodic spells" .)