Creature leaving sinking ship? Sailor returned (3)
I believe the answer is:
rat
'creature leaving sinking ship?' is the definition.
(as in 'rats leaving a sinking ship')
'sailor returned' is the wordplay.
'sailor' becomes 'tar' (tar can mean a sailor).
'returned' shows that the letters should be reversed in order.
'tar' written backwards gives 'RAT'.
(Other definitions for rat that I've seen before include "Desert - rodent" , "Renegade" , "Sewer rodent" , "one who deserts the sinking ship?" , "Contemptible person" .)