The compiler is on time, but essentially unprepared (9)
I believe the answer is:
impromptu
'unprepared' is the definition.
(similar in meaning)
'the compiler is on time but essentially' is the wordplay.
'the compiler' becomes 'I'm' (I've seen this in other clues).
'is on time' becomes 'prompt' (synonyms).
'essentially' indicates the central letters.
The central letter of 'but' is 'u'.
'im'+'prompt'+'u'='IMPROMPTU'
(Other definitions for impromptu that I've seen before include "Ad-lib" , "Ad hoc: spontaneous-sounding musical piece mastered by Schubert" , "On the spur of the moment" , "Unplanned; unrehearsed" , "With little or no preparation or forethought" .)