Put on a fortissimo number (5)
I believe the answer is:
affix
'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" .)