Apollo's instrument (4)
I believe the answer is:
lyre
'apollo's instrument' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lyre that I've seen before include "Rely (anag.)" , "Instrument played by Orpheus" , "Ancient musical instrument - sounds mendacious" , "Early harp" , "Plucked instrument" .)