Deserter from the artillery for a short time (3)
I believe the answer is:
rat
'deserter' is the definition.
(both can mean a traitor)
'artillery for a short time' is the wordplay.
'artillery' becomes 'RA' (Royal Artillery).
'for' means one lot of letters go next to another.
'a short time' becomes 't' (abbreviation; 'short' indicates a short form).
'ra'+'t'='RAT'
'from the' acts as a link.
(Other definitions for rat that I've seen before include "Betrayer" , "Destructive rodent" , "Tell-tale" , "Abandon" , "Long-tailed rodent" .)