Instrument is out of tune, but not initially (4)
I believe the answer is:
harp
'instrument' is the definition.
(I know that Harp is a instrument)
'out of tune but not initially' is the wordplay.
'out of tune' becomes 'sharp' (I've seen this in another clue).
'but not initially' means to remove the first letter.
'sharp' with its initial letter removed is 'HARP'.
'is' is the link.
(Other definitions for harp that I've seen before include "Instrument (for angels?)" , "Traditional Gaelic musical instrument" , "Keep on about" , "Instrument for angel" , "Lyre" .)