The old-fashioned piano? That's right (3)
I believe the answer is:
yep
'right' is the definition.
(thesaurus)
'the old-fashioned piano?' is the wordplay.
I cannot really see how this works, but
'piano?' could be 'p' (musical abbreviation) and 'p' is found within the answer.
'the old' could be 'ye' (resembles a historical spelling of 'the') and 'ye' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'that's' is the link.
Can you help me to learn more?
(Other definitions for yep that I've seen before include "informal approval" , "Correct" , "Positive reaction" , "Affirmative (US colloq.)" .)