Keep in check (7)
'keep in check' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for repress that I've seen before include "Keep in subjection" , "keep back" , "Forcibly keep down" , "Check" , "Put in irons, maybe" .)
(Other definitions for harness that I've seen before include "Horse's gear" , "Hitch up" , "Straps and fittings for horse control" , "Horse's reins, etc" , "Set of straps for horse or child" .)