Thin girl with stringed instrument (6)
I believe the answer is:
dilute
'thin' is the definition.
('dilute' can be a synonym of 'thin')
'girl with stringed instrument' is the wordplay.
'girl' becomes 'di'.
'with' says to put letters next to each other.
'stringed instrument' becomes 'lute' (lute is a kind of stringed instrument).
'di'+'lute'='DILUTE'
(Other definitions for dilute that I've seen before include "having less concentration" , "Reduce in strength or concentration" , "Adulterate" , "Reduce concentration of a liquid" , "The time when something ceases to have validity" .)