A number fit to send (5)
I believe the answer is:
cable
'send' is the definition.
(send a message by cable)
'a number fit' is the wordplay.
'a number' becomes 'C' (C is an example).
'fit' becomes 'able' (similar in meaning).
'c'+'able'='CABLE'
'to' is the link.
(Other definitions for cable that I've seen before include "Heavy chain or rope" , "Telegram; wire rope" , "Liberal Democrat" , "Wire, flex" , "Wire; message" .)