Chances characterising one leader of students (4)
I believe the answer is:
odds
'chances' is the definition.
(the odds of something happening)
'one leader of students' is the wordplay.
'one' becomes 'odd' (I've seen this before).
'leader of' says to take the initial letters.
The first letter of 'students' is 's'.
'odd'+'s'='ODDS'
'characterising' acts as a link.
(Other definitions for odds that I've seen before include "inequality" , "Measure of probability" , "ten to one?" , "The chances of something happening" , "at which one may be disagreeing" .)