Observe yours truly in audition? (3)
I believe the answer is:
eye
'observe' is the definition.
(eye can mean to observe or see)
'yours truly in audition?' is the wordplay.
'yours truly' becomes 'I' (the person that set this crossword).
'in audition?' shows a homophone (sound like) (audition can mean hearing).
'i' is a homophone of 'EYE'.
(Other definitions for eye that I've seen before include "See 1" , "Optic organ" , "Orb" , "'Public' or 'private' body part" , "Watch - middle of cyclone" .)