Female legitimately claimed to be ghastly (9)
I believe the answer is:
frightful
'ghastly' is the definition.
(thesaurus)
'female legitimately claimed' is the wordplay.
'female' becomes 'f' (abbreviation).
'legitimately claimed' becomes 'rightful' (I've seen this before).
'f'+'rightful'='FRIGHTFUL'
'to be' acts as a link.
(Other definitions for frightful that I've seen before include "ugly" , "from hell" , "Terrible, awful" , "horrible" , "ghastly" .)