In this sanctum it replaces clerical headgear (5)
I believe the answer is:
mitre
'clerical headgear' is the definition.
(I've seen this in another clue)
'in this sanctum it replaces' is the wordplay.
'in' indicates a hidden word.
'MITRE' can be found hidden inside 'this sanctum it replaces'.
(Other definitions for mitre that I've seen before include "Corner joint - bishop's hat" , "Bishop's cap - wood joint" , "Item of headgear" , "Timer (anag.)" , "Liturgical headdress" .)