Instrument not entirely problematic with piano (4)
I believe the answer is:
harp
'instrument' is the definition.
(harp is a kind of instrument)
'not entirely problematic with piano' is the wordplay.
'not entirely' means to remove the last letter.
'problematic' becomes 'hard' ('hard' can be similar in meaning to 'problematic').
'with' is a charade indicator (letters next to each other).
'piano' becomes 'p'.
'hard' with its last letter removed is 'har'.
'har'+'p'='HARP'
(Other definitions for harp that I've seen before include "Stringed instrument" , "Instrument for angel" , "Musical symbol" , "Emblem of Ireland" , "Instrument (for angels?)" .)