Instrument out of tune, but not initially (4)
I believe the answer is:
harp
'instrument' is the definition.
(harp is a kind of instrument)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for harp that I've seen before include "Lyre" , "Heavenly instrument?" , "''The ... that once through Tara's halls'' (4)" , "Instrument (for angels?)" , "Musical symbol" .)