Key period not entirely wasted, ready for review (9)
I believe the answer is:
alterable
'ready for review' is the definition.
(able to be altered or reviewed)
'key period not entirely wasted' is the wordplay.
I cannot quite understand how this works, but
'key' could be 'alt' (the Alt keyboard button) and 'alt' is found within the answer.
'period' could be 'era' (era is a kind of period) and 'era' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for alterable that I've seen before include "likely to change" , "Subject to change" .)