Best quality piano introducing old poem (5)
I believe the answer is:
prime
'best quality' is the definition.
(prime means the best quality)
'piano introducing old poem' is the wordplay.
'piano' becomes 'p' (musical abbreviation).
'introducing' says to put letters next to each other.
'old poem' becomes 'rime' (archaic spelling of 'rhyme').
'p'+'rime'='PRIME'
(Other definitions for prime that I've seen before include "Best quality, perhaps of beef" , "Prepare to set off" , "Number with no factor but itself and 1" , "Maybe eleven" , "Prepare to fire - first" .)