Weaken woman with instrument (6)
I believe the answer is:
dilute
'weaken' is the definition.
(I know that dilute is a more specific form of the action the weaken)
'woman with instrument' is the wordplay.
'woman' becomes 'Di' (short for Diana).
'with' means one lot of letters go next to another.
'instrument' becomes 'lute' (lute is a kind of instrument).
'di'+'lute'='DILUTE'
(Other definitions for dilute that I've seen before include "Adulterate" , "Reduce in strength or concentration" , "Thin down by adding water" , "The time when something ceases to have validity" , "Weaken by adding water" .)