Not a musical score (6)
I believe the answer is:
twenty
'score' is the definition.
(synonyms)
'not a musical' is the wordplay.
I cannot quite understand how this works, but
'not' could be 'nt' (synonyms) and 'nt' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for twenty that I've seen before include "Score, XX" , "Number" , "No. between 5 and 1 on dartboard" , "No longer a teenager" , "Hundredweights to a ton" .)