I am on time; you sound unprepared (9)
I believe the answer is:
impromptu
'unprepared' is the definition.
(impromptu means improvised or unprepared)
'i am on time you sound' is the wordplay.
'i am' becomes 'I'm'.
'on time' becomes 'prompt' (synonyms).
'sound' shows a homophone (sound like).
'you' is a homophone of 'u'.
'im'+'prompt'+'u'='IMPROMPTU'
(Other definitions for impromptu that I've seen before include "Piano piece" , "Ad-lib" , "Without much planning" , "unrehearsed piece" , "Ad hoc: spontaneous-sounding musical piece mastered by Schubert" .)