Thin little girl with musical instrument (6)
I believe the answer is:
dilute
'thin' is the definition.
('dilute' can be a synonym of 'thin')
'little girl with musical instrument' is the wordplay.
'little girl' becomes 'di' (short for Diane, Diana etc).
'with' means one lot of letters go next to another.
'musical instrument' becomes 'lute' (lute is a kind of musical instrument).
'di'+'lute'='DILUTE'
(Other definitions for dilute that I've seen before include "Thin down by adding water" , "Add liquid to lessen strength" , "Reduce in strength or concentration" , "Add water to thin" , "Tone down" .)