A cross can't somehow be holy (10)
I believe the answer is:
sacrosanct
'holy' is the definition.
(thesaurus)
'a cross can't somehow' is the wordplay.
'somehow' is an anagram indicator.
'a'+'cross'+'cant'='acrosscant'
'acrosscant' with letters rearranged gives 'SACROSANCT'.
'be' acts as a link.
(Other definitions for sacrosanct that I've seen before include "above criticism" , "Too valuable to be changed" , "Unchallengeable" , "Revered, untouchable" , "Co-stars can (anag.)" .)