Terrible score for Hearts (5)
I believe the answer is:
cores
'for hearts' is the definition.
(core can mean the heart of something)
'terrible score' is the wordplay.
'terrible' indicates anagramming the letters.
'score' with letters rearranged gives 'CORES'.
(Other definitions for cores that I've seen before include "Central parts" , "Hearts" , "The centres of objects" .)