Viewer, the first person in auditorium (3)
I believe the answer is:
eye
'viewer' is the definition.
(I've seen this before)
'the first person in auditorium' is the wordplay.
'the first person' becomes 'i' ('I' is a first-person pronoun in grammar).
'in auditorium' indicates a 'sounds like' (homophone) clue (as the word might sound to an audience).
'i' sounds like 'EYE'.
(Other definitions for eye that I've seen before include "Organ of vision" , "Symmetrical spot" , "Sense organ" , "Location of 11dn" , "Centre of storm" .)