Try to accommodate teachers' union, if that's what's most desirable (8)
I believe the answer is:
tastiest
'that's what's most desirable' is the definition.
I can't judge whether this definition defines the answer.
'try to accommodate teachers' union' is the wordplay.
'try' becomes 'test' (test is a kind of try).
'to accommodate' is an insertion indicator.
'teachers' union' becomes 'asti' (I am not sure about this - if you are sure you should believe this answer much more).
'test' enclosing 'asti' is 'TASTIEST'.
'if' acts as a link.
Can you help me to learn more?
(Other definitions for tastiest that I've seen before include "Most savoury and toothsome" , "Most flavoursome" , "It's most pleasant" , "Most delicious" .)