It's grand for a boy to be musical (4)
I believe the answer is:
song
'be musical' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'it's grand for a boy' is the wordplay.
'it's grand' becomes 'g' (synonyms).
'for' says to put letters next to each other (I've seen this in other clues).
'a boy' becomes 'son' ('son' can be a synonym of 'boy').
'g' after 'son' is 'SONG'.
'to' is the link.
Can you help me to learn more?
(Other definitions for song that I've seen before include "Piece of music, usually with words" , "Vocal composition" , "Musical composition with words" , "Ballad, aria" , "Lied" .)