Music ultimately reaches heart (5)
I believe the answer is:
score
'music' is the definition.
(score is a kind of music)
'ultimately reaches heart' is the wordplay.
'ultimately' indicates one should take the final letters.
'heart' becomes 'core' ('core' can be a synonym of 'heart').
The final letter of 'reaches' is 's'.
's'+'core'='SCORE'
(Other definitions for score that I've seen before include "Music from film" , "Tally; group of twenty" , "Put ball in the net" , "Group of 20" , "Musical composition in written form" .)