Carefree and pleased to leave fortune to unknown quantity (5-2-5)
I believe the answer is:
happy-go-lucky
'carefree' is the definition.
(I know that carefree can be written as happy-go-lucky)
'pleased to leave fortune to unknown quantity' is the wordplay.
'pleased' becomes 'happy' (pleased means contented).
'to leave' becomes 'go' (both can mean to depart).
'fortune' becomes 'luck' (synonyms).
'to unknown quantity' becomes 'y' (algebra).
'happy'+'go'+'luck'+'y'='HAPPY-GO-LUCKY'
'and' acts as a link.
(Other definitions for happy-go-lucky that I've seen before include "Cheerfully irresponsible" , "Cheerfully unconcerned about the future" , "Insouciant" , "Free and easy" , "Carefree" .)