Awfully fine legend that is impossible to describe (9)
I believe the answer is:
ineffable
'is impossible to describe' is the definition.
(I've seen this before)
'awfully fine legend' is the wordplay.
'awfully' is an anagram indicator.
'legend' becomes 'fable' (synonyms).
'fine' is an anagram of 'inef'.
'inef'+'fable'='INEFFABLE'
'that' acts as a link.
(Other definitions for ineffable that I've seen before include "Unutterable, too great to be described" , "Indescribable in words" , "defying expression" , "Defying description" , "Too sacred to be uttered" .)