Crossword Genius

In building a score, they're heartening (5)

Ross

I believe the answer is:

cores

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

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