Setter is ready to act, solver textually unprepared (9)
I believe the answer is:
impromptu
'unprepared' is the definition.
(impromptu means improvised or unprepared)
'setter is ready to act solver' is the wordplay.
'setter is' becomes 'I'm' (the setter is the person who set the crossword).
'ready' becomes 'prompt' (similar in meaning).
'to act solver' becomes 'u' (I can't explain this - if you can you should believe this answer much more).
'im'+'prompt'+'u'='IMPROMPTU'
'textually' is the link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for impromptu that I've seen before include "Without rehearsal or preparation" , "Unprepared" , "Without much planning" , "Without pre-planning" , "piece by Chopin, perhaps" .)