One to watch in the examinations? (11)

'one' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

'to watch in the examinations?' is the wordplay.
I cannot quite understand how this works, but
'to' is present in the answer.
'watch' could be 'vigil' ('vigil' can be a synonym of 'watch') and 'vigil' is present in the answer.
'in' is within the answer.
The remaining letters 'ar' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...