....upset one's performance after beginning to doubt (10)
I believe the answer is:
disconcert
'upset' is the definition.
(disconcerting is a kind of upseting)
'one's performance after beginning to doubt' is the wordplay.
'one's' becomes 'is' (roman numeral).
'performance' becomes 'concert' (concert is a kind of performance**).
'after' says to put letters next to each other.
'beginning to' says to take the initial letters.
The initial letter of 'doubt' is 'd'.
'is'+'concert'='isconcert'
'isconcert' after 'd' is 'DISCONCERT'.
(Other definitions for disconcert that I've seen before include "Dismay" , "Throw" , "Disturb, unsettle" , "Confuse; worry" , "Bother" .)