The writer's cue initially unthinkingly unscripted (9)
I believe the answer is:
impromptu
'unscripted' is the definition.
(I've seen this before)
'the writer's cue initially unthinkingly' is the wordplay.
'the writer's' becomes 'im' (both could mean 'the crossword setter is').
'cue' becomes 'prompt' (prompt is a kind of cue).
'initially' suggests taking the first letters.
The first letter of 'unthinkingly' is 'u'.
'im'+'prompt'+'u'='IMPROMPTU'
(Other definitions for impromptu that I've seen before include "Ad-lib" , "Unplanned; unrehearsed" , "Spontaneous" , "Improvised" , "Without rehearsal or preparation" .)