Tees had turned old instrument tone down (6)
I believe the answer is:
dilute
'tone down' is the definition.
(I've seen this before)
'tees had turned old instrument' is the wordplay.
'tees had' becomes 'I'd' (where the crossword setter is 'Tees').
'turned' says the letters should be written backwards.
'old instrument' becomes 'lute' (medieval stringed instrument).
'id' reversed gives 'di'.
'di'+'lute'='DILUTE'
(Other definitions for dilute that I've seen before include "Add liquid to lessen strength" , "Adulterate" , "Tone down" , "Add water to another liquid" , "Thin out, make less strong" .)