First-class one-liner (5)
I believe the answer is:
crack
'first-class one-liner' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Maybe there's an association between them I don't understand?
'first-class one-liner' is the wordplay.
'first class' becomes 'c' (rst letter of 'class').
'one-liner' becomes 'rack' (I can't justify this - if you can you should give a lot more credence to this answer).
'c'+'rack'='CRACK'
Can you help me to learn more?
(Other definitions for crack that I've seen before include "Narrow opening; joke" , "Narrow opening, break" , "Take severe measures - . . . . . down on" , "Try" , "Witticism" .)