Rather entertaining piano is medium for sound transposition (10)
I believe the answer is:
spoonerism
'sound transposition' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'rather entertaining piano is medium' is the wordplay.
'rather' becomes 'sooner' ('I'd sooner do that' means 'I'd rather do that').
'entertaining' indicates putting letters inside (inviting in, as for dinner).
'piano' becomes 'p' (musical abbreviation).
'medium' becomes 'm' (as in UK clothing sizes).
'sooner' going around 'p' is 'spooner'.
'spooner'+'is'+'m'='SPOONERISM'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for spoonerism that I've seen before include "Verbal exchange of leading characters" , ""A lopping of sweaters", for example" , "Regular announcement by old clergyman" , "Burly chassis for Shirley Bassey, say?" , "A slip of the tongue? -- no promises (anag)" .)