Marshal sacred to old soldiers (8)
I believe the answer is:
redcoats
'old soldiers' is the definition.
'marshal sacred to' is the wordplay.
'marshal' indicates an anagram (I've seen 'marshals' mean this (marshal can mean to gather)).
'sacred'+'to'='sacredto'
'sacredto' is an anagram of 'REDCOATS'.
(Other definitions for redcoats that I've seen before include "men who used to fight for Britain" , "Old British soldiers in colourful uniforms" , "British soldiers during the American revolution" .)