Key period almost squandered, offering chance to change (9)
I believe the answer is:
alterable
'key' is the definition.
Both the definition and answer are adjectives. Maybe you can see an association between them that I can't see?
'period almost squandered offering chance to change' is the wordplay.
'period' becomes 'ab' (Ab is an example).
'almost squandered offering chance' becomes 'le' (I am not sure about this - if you are sure you should believe this answer much more).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'change' becomes 'alter' ('alter' can be a synonym of 'change').
'ab'+'le'='able'
'able' after 'alter' is 'ALTERABLE'.
Can you help me to learn more?
(Other definitions for alterable that I've seen before include "Subject to change" , "likely to change" .)