Short kid in need of a new heart (5)
I believe the answer is:
terse
'short' is the definition.
(terse can mean short or brusque)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for terse that I've seen before include "Short and succinct" , "precise" , "Using few words" , "Curt, laconic of speech" , "Abrupt, concise" .)