Vocalist missing intro holds note in number (7)
I believe the answer is:
integer
'number' is the definition.
(I know that integer is a type of real number)
'vocalist missing intro holds note' is the wordplay.
'vocalist' becomes 'singer' (synonyms).
'missing intro' means to remove the first letter.
'holds' indicates putting letters inside.
'note' becomes 'te' (musical note).
'singer' with its first letter removed is 'inger'.
'inger' placed around 'te' is 'INTEGER'.
'in' acts as a link.
(Other definitions for integer that I've seen before include "A whole number, not a fraction" , "Positive or negative whole number" , "Gin tree (anag)" , "Result of counting" , "Get Erin (anag)" .)