In building a score, they're heartening (5)
I believe the answer is:
cores
'in building a' is the definition.
I can't tell whether this defines the answer.
'score they're heartening' is the wordplay.
'score' anagrams to 'CORES'.
Though, I'm not clear how the anagram is indicated.
Can you help me to learn more?
(Other definitions for cores that I've seen before include "The centres of objects" , "Central parts" , "Hearts" .)