Little possibility of losing weight (3,6)
I believe the answer is:
fat chance
'losing weight' is the definition.
I don't know anything about this answer so I cannot tell whether this works.
'little possibility' is the wordplay.
'little' becomes 'fat' (I can't explain this - if you can you should give a lot more credence to this answer).
'possibility' becomes 'chance' (chance is a kind of possibility).
'fat'+'chance'='FAT CHANCE'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for fat chance that I've seen before include "Little prospect" , "Slim hope, paradoxically!" , "that won't happen" , "It will never happen!" , "Slender hope (odd though that sounds)" .)