Chubby lad's bottom almost all visible in cast off (3,2,3)
I believe the answer is:
put to sea
'cast off' is the definition.
(both can mean to begin a sea voyage)
'chubby lad's bottom almost all' is the wordplay.
'chubby lad' becomes 'putto' (type of chubby cherub in Renaissance art).
'bottom' becomes 'seat' (synonyms).
'almost all' means to remove the last letter (most but not all of the word).
'seat' with its last letter taken off is 'sea'.
'putto'+'sea'='PUT TO SEA'
'visible in' is the link.
(Other definitions for put to sea that I've seen before include "start on the drink" , "Sail away" , "Set sail" .)