Deserter from the artillery for a short time (3)
I believe the answer is:
rat
'deserter' is the definition.
(thesaurus)
'the artillery for a short time' is the wordplay.
'the artillery' becomes 'ra' (Royal Artillery).
'for' is a charade indicator (letters next to each other).
'a short time' becomes 't' (abbreviation for time).
'ra'+'t'='RAT'
'from' acts as a link.
(Other definitions for rat that I've seen before include "Betrayer" , "Destructive rodent" , "Tell-tale" , "Abandon" , "Long-tailed rodent" .)