Deserter? Not in the desert once! (3)
I believe the answer is:
rat
'deserter?' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
(Other definitions for rat that I've seen before include "One deserting sinking ship" , "Rodent; traitor" , "Scumbag" , "Change sides" , "Rodent - deserter" .)