Large gamble? That's highly unlikely (3,6)
I believe the answer is:
fat chance
'highly unlikely' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'large gamble?' is the wordplay.
'large' becomes 'fat' (I've seen this in other clues).
'gamble?' becomes 'chance' (synonyms).
'fat'+'chance'='FAT CHANCE'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for fat chance that I've seen before include "that won't happen" , "It will never happen!" , "Little prospect" , "No likelihood at all!" , "Slender hope (odd though that sounds)" .)