Jack backs rate reduction (3)
I believe the answer is:
tar
'jack' is the definition.
(both are informal terms for sailor)
'backs rate reduction' is the wordplay.
'backs' says the letters should be written backwards.
'reduction' means to remove the last letter.
'rate' with its final letter taken off is 'rat'.
'rat' reversed gives 'TAR'.
(Other definitions for tar that I've seen before include "Pitch; sailor" , "Sticky stuff for a kind of sailor" , "Sticky road material or informal sailor" , "Black coat" , "Product of smoking cigarettes" .)