Marshal sacred to old soldiers (8)

I believe the answer is:

redcoats

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'old soldiers' is the definition.

'marshal sacred to' is the wordplay.
'sacred to' anagrams to 'REDCOATS'.
But, I am not clear how the anagram is indicated.

Can you help me to learn more?

(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" .)

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