I'd turned medieval guitar's tone down (6)
I believe the answer is:
dilute
'tone down' is the definition.
(I've seen this before)
'i'd turned medieval guitar's' is the wordplay.
'turned' shows that the letters should be reversed in order.
'medieval guitar' becomes 'lute' (I've seen this in another clue).
'id' in reverse letter order is 'di'.
'di'+'lute'='DILUTE'
(Other definitions for dilute that I've seen before include "The time when something ceases to have validity" , "having less concentration" , "Weaken by adding water" , "Add water to another liquid" , "Adulterate" .)