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" .)

I've seen this clue in The Telegraph, The Guardian and The Globe and Mail.

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