An act with cross somehow regarded as holy (10)
I believe the answer is:
sacrosanct
'holy' is the definition.
(thesaurus)
'an act with cross somehow' is the wordplay.
'with' means one lot of letters go next to another.
'somehow' indicates an anagram.
'an'+'act'='anact'
'anact' after 'cross' is 'crossanact'.
'crossanact' is an anagram of 'SACROSANCT'.
'regarded as' is the link.
(Other definitions for sacrosanct that I've seen before include "Too valuable to touch" , "Revered, untouchable" , "Too important to touch" , "Co-stars can (anag.)" , "above criticism" .)