Old soldier heading off demonstrator (6)
I believe the answer is:
archer
'old soldier' is the definition.
'heading off demonstrator' is the wordplay.
'heading off' means to remove the first letter.
'demonstrator' becomes 'marcher' (I've seen this before).
'marcher' with its first letter removed is 'ARCHER'.
(Other definitions for archer that I've seen before include "Tell perhaps" , "part of circle woman's joined" , "Tell possibly < more mischievous" , "Sagittarius - bowman" , "old jockey" .)