Towering form of ominous aunt (11)
I believe the answer is:
mountainous
'towering' is the definition.
(thesaurus)
'form of ominous aunt' is the wordplay.
'form of' indicates an anagram (the letters need to be in another 'form').
'ominous'+'aunt'='ominousaunt'
'ominousaunt' is an anagram of 'MOUNTAINOUS'.
(Other definitions for mountainous that I've seen before include "Of very high hills; huge" , "With high hills and crags" , "Having many peaks" .)