Sign on score from Uncle Fred (4)
I believe the answer is:
clef
'sign on score' is the definition.
'clef' can be an answer for 'sign' (I have seen 'Sign of pitch' mean 'clef' so perhaps 'sign' could also mean 'clef'). I am unsure of the 'on score' bit.
'from uncle fred' is the wordplay.
'from' says the answer is hidden in the clue.
'CLEF' is hidden in the letters of 'uncle fred'.
Can you help me to learn more?
(Other definitions for clef that I've seen before include "Musical notation on a staff" , "Musical key or stave" , "Musical pitch notation" , "Item of musical notation" , "symbol used by 6 [VERDI]" .)