Aching to retain a hundred notes (5)
I believe the answer is:
score
'notes' is the definition.
(musical score)
'aching to retain a hundred' is the wordplay.
'aching' becomes 'sore' (with aches).
'to retain' indicates putting letters inside.
'a hundred' becomes 'c' (Roman numeral).
'sore' placed around 'c' is 'SCORE'.
(Other definitions for score that I've seen before include "Reckoning" , "Chalk up - twenty" , "Group of 20" , "Music from film" , "Mark; orchestrate" .)