Fifty per cent chance of drunk returning drink (4-2)
I believe the answer is:
toss-up
'fifty per cent chance' is the definition.
I can't judge whether this definition defines the answer.
'drunk returning drink' is the wordplay.
'drunk' becomes 'sot' ('sot' can be a synonym of 'drunk').
'returning' says the letters should be written backwards.
'drink' becomes 'sup' (I've seen this before).
'sot' written backwards gives 'tos'.
'tos'+'sup'='TOSS-UP'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for toss-up that I've seen before include "Flip of a coin" , "Fifty-fifty chance" , "It helps you pick" , "Use of a coin to choose" .)