Judge a piano flatter? (8)
I believe the answer is:
appraise
'judge' is the definition.
(appraising is a kind of judging)
'a piano flatter?' is the wordplay.
'piano' becomes 'p' (musical abbreviation).
'flatter?' becomes 'praise' (flattering is a kind of praising).
'a'+'p'+'praise'='APPRAISE'
(Other definitions for appraise that I've seen before include "Estimate or assess the value of" , "Assess (a thing's) quality" , "Tip off" , "Judge" , "Make assessment of value or quality" .)