Weaken woman with stringed 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 stringed instrument' is the wordplay.
'woman' becomes 'Di' (short for Diana).
'with' is a charade indicator (letters next to each other).
'stringed instrument' becomes 'lute' (lute is a type of stringed instrument).
'di'+'lute'='DILUTE'
(Other definitions for dilute that I've seen before include "Add liquid to lessen strength" , "Adulterate" , "Weaken by adding water" , "Thin down by adding water" , "Reduce concentration of a liquid" .)