Chances of winning toss in event — not entirely small (5)
I believe the answer is:
evens
'chances of winning toss' is the definition.
'evens' can be an answer for 'chances' (I've seen this before). I'm unsure of the 'of winning toss' bit.
'event not entirely small' is the wordplay.
'not entirely' means to remove the last letter (not all of the word).
'small' becomes 's' (abbreviation - e.g. clothes size).
'event' with its final letter removed is 'even'.
'even'+'s'='EVENS'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for evens that I've seen before include "square" , "50-50 chance" , "less likely than 27 [ODDS-ON]" , "In betting, winning the amount staked" , "These are odds, strangely" .)