Sailor putting scoundrel up (3)
I believe the answer is:
tar
'sailor' is the definition.
(tar is a kind of sailor)
'scoundrel up' is the wordplay.
'scoundrel' becomes 'rat' (I've seen this in other clues).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'rat' in reverse letter order is 'TAR'.
'putting' acts as a link.
(Other definitions for tar that I've seen before include "not put on ship in proverbial case of false economy?" , "one may need to pay on deck" , "used by those who pay to work on deck" , "Black coat" , "Sticky cigarette residue" .)