Holy order across a northern court (10)
I believe the answer is:
sacrosanct
'holy' is the definition.
(thesaurus)
'order across a northern court' is the wordplay.
'order' indicates anagramming the letters.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'court' becomes 'ct' (abbreviation used in road names).
'across' anagrammed gives 'sacros'.
'sacros'+'a'+'n'+'ct'='SACROSANCT'
(Other definitions for sacrosanct that I've seen before include "Inviolable" , "Unchallengeable" , "Too important to touch" , "above criticism" , "Co-stars can (anag.)" .)