Join in a very loud number (5)
I believe the answer is:
affix
'join' is the definition.
(I've seen this before)
'a very loud number' is the wordplay.
'very loud' becomes 'ff' (musical abbreviation).
'number' becomes 'IX' (IX is an example).
'a'+'ff'+'ix'='AFFIX'
'in' is the link.
(Other definitions for affix that I've seen before include "Join" , "Fasten (to something)" , "Stick on" , "Glue" , "Attach" .)