Letter from Greece written by an old music producer (5)
I believe the answer is:
piano
'music producer' is the definition.
I know that 'music' relates to this answer. I also know that both the answer and definition are singular nouns
'letter from greece written by an old' is the wordplay.
'letter from greece' becomes 'pi' (letter from the Greek alphabet).
'written by' is a charade indicator (letters next to each other).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'pi'+'an'+'o'='PIANO'
Can you help me to learn more?
(Other definitions for piano that I've seen before include "p (mus.)" , "Softly (mus.)" , "Musical instrument - play quietly" , "Musical instrument - softly, please" , "Direction in music (1): relatively softly" .)