Weak instrument managed without (7)
I believe the answer is:
diluted
'weak' is the definition.
(similar in meaning)
'instrument managed without' is the wordplay.
'instrument' becomes 'lute' (lute is a kind of instrument).
'managed' becomes 'did' ('do' can mean 'manage' as in 'do without').
'without' indicates putting letters inside.
'lute' placed inside 'did' is 'DILUTED'.
(Other definitions for diluted that I've seen before include "reduced in strength" , "Weaker" , "Watered down or attenuated." , "Lessened the strength of a solution" .)