Old soldier: marshal or cadet (7)
I believe the answer is:
redcoat
'old soldier' is the definition.
(I've seen this before)
'marshal or cadet' is the wordplay.
'marshal' indicates an anagram (I've seen 'marshals' mean this (marshal can mean to gather)).
'or'+'cadet'='orcadet'
'orcadet' anagrammed gives 'REDCOAT'.
(Other definitions for redcoat that I've seen before include "Do react (anag) - old soldier" , "British soldier in the American War of Independence" , "British soldier or Butlin's worker" , "Old-time British soldier" , "Former British soldier" .)