The compiler is ready -- solver reportedly unprepared (9)
I believe the answer is:
impromptu
'unprepared' is the definition.
('impromptu' can be similar in meaning to 'unprepared')
'the compiler is ready solver reportedly' is the wordplay.
'the compiler' becomes 'im' (I've seen this in other clues).
'is ready' becomes 'prompt' ('prompt' can be similar in meaning to 'ready'**).
'solver' becomes 'you' (the person solving the crossword).
'reportedly' indicates a 'sounds like' (homophone) clue.
'you' sounds like 'u'.
'im'+'prompt'+'u'='IMPROMPTU'
(Other definitions for impromptu that I've seen before include "Unplanned; instrumental piece" , "Unplanned; composition" , "Unscripted" , "Ad hoc: spontaneous-sounding musical piece mastered by Schubert" , "Without pre-planning" .)