Communication from a magician about a letter (7)
I believe the answer is:
message
'communication' is the definition.
(a message is something communicated)
'a magician about a letter' is the wordplay.
'a magician' becomes 'mage'.
'about' means one lot of letters goes inside another.
'a letter' becomes 'ess' (the letter S).
'mage' enclosing 'ess' is 'MESSAGE'.
'from' is the link.
(Other definitions for message that I've seen before include "Communication that is sent" , "The medium is the - (McLuhan)" , "Spoken or written communication" , "Verbal or written communication" , "Signal" .)