Sun's rabid support was taken for granted (6)
I believe the answer is:
unsaid
'taken for granted' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I can't see?
'sun's rabid support' is the wordplay.
'rabid' indicates an anagram.
'support' becomes 'aid' (to aid is to help or support).
'sun' with letters rearranged gives 'uns'.
'uns'+'aid'='UNSAID'
'was' acts as a link.
Can you help me to learn more?
(Other definitions for unsaid that I've seen before include "omitted from speech" , "left to the imagination" , "Not spoken" , "not delivered" , "Not made explicit" .)