Number is loud, if adolescent (7)
I believe the answer is:
fifteen
'number' is the definition.
(fifteen is a kind of whole number)
'loud if adolescent' is the wordplay.
'loud' becomes 'f' (short for forte - 'loud' in music).
'adolescent' becomes 'teen' (teenager).
'f'+'if'+'teen'='FIFTEEN'
'is' is the link.
(Other definitions for fifteen that I've seen before include "Film type" , "Product of two primes" , "... number" , "Square root of 225" , "/8 across Today's milestone. That's a lot of Simplex crosswords" .)