The first gag could be what unnerves performer (5,6)
I believe the answer is:
stage fright
'what unnerves performer' is the definition.
(stage fright makes a performer nervous)
'the first gag could be' is the wordplay.
'could be' indicates anagramming the letters.
'the'+'first'+'gag'='thefirstgag'
'thefirstgag' is an anagram of 'STAGE-FRIGHT'.
(Other definitions for stage fright that I've seen before include "Nerves in front of an audience" , "Pre-performance nerves" , "state of nerves" , "Performer's fear" , "Dramatic nerves" .)