Rodent sailor's turned back (3)
I believe the answer is:
Here is my best explanation:
'rodent' is the definition.
(rat is a kind of rodent)
'sailor's turned back' is the subsidiary indication.
'sailor' becomes 'tar' (tar is a kind of sailor).
'turned back' says the letters should be written in reverse.
'tar' in reverse letter order is 'rat'.
(Other definitions for rat that I've seen before include "Scumbag", "Rodent; traitor", "Sewer rodent", "Cad", "Betray like a rodent", "Destructive rodent", "Inform", "Treacherous rodent", "Rodent pest" and "Despicable person".)