Jack backs rate reduction (3)
I believe the answer is:
tar
'jack' is the definition.
(thesaurus)
'backs rate reduction' is the wordplay.
'backs' is a reversal indicator (I've seen 'back' mean this).
'reduction' means to remove the last letter.
'rate' with its last letter taken off is 'rat'.
'rat' back-to-front is '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" .)