Sailor turned rodent (3)
I believe the answer is:
tar
'sailor' is the definition.
(slang term for a sailor)
'turned rodent' is the wordplay.
'turned' says the letters should be written in reverse.
'rodent' becomes 'rat' (rat is a kind of rodent).
'rat' written backwards gives 'TAR'.
(Other definitions for tar that I've seen before include "Informal sailor or road surfacing material" , "Informal sailor, the rat" , "It may cover the road" , "Hearty" , "Dark viscous flammable substance" .)