Scoundrel Jack set up (3)
I believe the answer is:
rat
'scoundrel' is the definition.
(rat can mean a nasty person)
'jack set up' is the wordplay.
'jack' becomes 'tar' (both are informal terms for sailor).
'set up' says the letters should be written in reverse (in a down clue, letters go up).
'tar' reversed gives 'RAT'.
(Other definitions for rat that I've seen before include "Tell-tale" , "Despicable person" , "shopper" , "Cad" , "Desert - rodent" .)