Old soldier - or cadet being trained (7)
I believe the answer is:
redcoat
'old soldier' is the definition.
(I've seen this before)
'or cadet being trained' is the wordplay.
'being trained' indicates anagramming the letters (train can mean to shape or manipulate).
'or'+'cadet'='orcadet'
'orcadet' anagrammed gives 'REDCOAT'.
(Other definitions for redcoat that I've seen before include "Former British soldier" , "Old-time British soldier" , "British soldier or Butlin's worker" , "Tommy, once" , "British soldier (hist.)" .)