Appraise girl, one probing stringed instrument (8)
I believe the answer is:
evaluate
'appraise' is the definition.
(I know this)
'girl one probing stringed instrument' is the wordplay.
'girl' becomes 'Eva'.
'one' becomes 'a' (a thing is one thing).
'probing' means one lot of letters goes inside another (probe can mean to make a hole in).
'stringed instrument' becomes 'lute' (lute is a kind of stringed instrument).
'a' inserted into 'lute' is 'luate'.
'eva'+'luate'='EVALUATE'
(Other definitions for evaluate that I've seen before include "Make an assessment of, determine worth" , "'Assess, appraise (8)'" , "Reckon worth of" , "Judge the worth of something" , "Make an assessment of, measure" .)