Sustained note in number just now oddly falling away (6)
I believe the answer is:
tenuto
'sustained note' is the definition.
(I've seen this before)
'number just now oddly falling away' is the wordplay.
'number' becomes 'ten' (ten is a kind of number).
'oddly falling away' indicates alternate letters.
'just'+'now'='justnow'
The alternate letters of 'justnow' are 'uto'.
'ten'+'uto'='TENUTO'
'in' acts as a link.
(Other definitions for tenuto that I've seen before include "held in Jerusalem?" , "(Of a note) sustained" , "Held for its full time value (music)" , "Of musical note held for its full value" , "held in music class" .)