Vocalist misses start, holding note in number (7)
I believe the answer is:
integer
'number' is the definition.
(I know that integer is a type of number)
'vocalist misses start holding note' is the wordplay.
'vocalist' becomes 'singer' (synonyms).
'misses start' means to remove the first letter (I've seen 'missing start' mean this).
'holding' means one lot of letters goes inside another.
'note' becomes 'te' (musical note as in do re mi fa so la ti).
'singer' with its first letter taken off 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" , "Treeing (anag.)" , "Perhaps 60,000" , "One for example" , "Whole unit" .)