He’s well up in music! (4)
I believe the answer is:
alto
'music' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Maybe there's a link between them I don't understand?
'he's well up' is the wordplay.
'he' becomes 'Al'.
'well up' becomes 'to' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'al'+'to'='ALTO'
'in' is the link.
Can you help me to learn more?
(Other definitions for alto that I've seen before include "Kind of singing voice, male or female" , "High male singing voice" , "Highest male voice: lowest female voice" , "Voice part" , "The highest adult male singing voice" .)