Did accept instrument was weakened (7)
I believe the answer is:
diluted
'weakened' is the definition.
(similar in meaning)
'did accept instrument' is the wordplay.
'accept' means one lot of letters goes inside another.
'instrument' becomes 'lute' (lute is a kind of instrument).
'did' enclosing 'lute' is 'DILUTED'.
'was' is the link.
(Other definitions for diluted that I've seen before include "Lessened the strength of a solution" , "Weaker" , "reduced in strength" , "Watered down or attenuated." .)