Tense, as is pupil in audition (4)
I believe the answer is:
taut
'tense' is the definition.
(similar in meaning)
'as is pupil in audition' is the wordplay.
'as is pupil' becomes 'taught' (a pupil is taught at school).
'in audition' indicates a 'sounds like' (homophone) clue (audition can mean hearing).
'taught' sounds like 'TAUT'.
(Other definitions for taut that I've seen before include "Stretched tight or disciplined" , "Tense, stretched tight" , "Stretched, pulled tight" , "Pulled or drawn tight" , "On the slope" .)