Sign on staff, almost split (4)
I believe the answer is:
clef
'sign on staff' 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'm unsure of the 'on staff' bit.
'almost split' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'split' becomes 'cleft' (cleft can mean split or cloven).
'cleft' with its last letter removed is 'CLEF'.
Can you help me to learn more?
(Other definitions for clef that I've seen before include "Symbol in musical score showing pitch of the notes" , "Treble or bass symbol?" , "Musical pitch notation" , "Musical notation on a staff" , "Symbol on the left of a musical stave" .)