Rodent sailor’s turned back (3)
I believe the answer is:
rat
'rodent' is the definition.
(rat is a kind of rodent)
'sailor's turned back' is the wordplay.
'sailor' becomes 'tar' (tar is a kind of sailor).
'turned back' is a reversal indicator.
'tar' in reverse letter order is 'RAT'.
(Other definitions for rat that I've seen before include "Contemptible person" , "Destructive rodent" , "one not going down with ship" , "Plague-carrying rodent" , "Betray like a rodent" .)