Retreating sailor, a deserter (3)
I believe the answer is:
rat
'a deserter' is the definition.
(thesaurus)
'retreating sailor' is the wordplay.
'retreating' says the letters should be written in reverse.
'sailor' becomes 'tar' (tar is a kind of sailor).
'tar' in reverse letter order is 'RAT'.
(Other definitions for rat that I've seen before include "Contemptible person" , "Desert - rodent" , "one escaping doomed vessel?" , "Grass" , "Destructive rodent" .)