Crossword Genius

Put on a fortissimo number (5)

Ross

I believe the answer is:

affix

I'm a little stuck... Click here to teach me more about this clue!

'put' is the definition.
The answer and definition can be both to do with contact as well as being verbs in their base form.
Maybe you can see a link between them that I can't see?

'on a fortissimo number' is the wordplay.
'on' is a charade indicator (letters next to each other).
'fortissimo' becomes 'ff'.
'number' becomes 'ix' (IX is an example**).
'a'+'ff' is 'aff'.
'aff'+'ix'='AFFIX'

Can you help me to learn more?

(Other definitions for affix that I've seen before include "Glue" , "Join" , "Attach" , "Append" , "Stick on" .)

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