Unusual second chances (4)
I believe the answer is:
odds
'chances' is the definition.
(the odds of something happening)
'unusual second' is the wordplay.
'unusual' becomes 'odd' (similar in meaning).
'second' becomes 's' (common abbreviation).
'odd'+'s'='ODDS'
(Other definitions for odds that I've seen before include "(In betting) chances" , "Scraps" , "familiar to our betters?" , "Indicator of likely betting winnings" , "Betting price" .)