Instrument required by Simply Red (4)
I believe the answer is:
lyre
'instrument' is the definition.
(lyre is a kind of instrument)
'required by simply red' is the wordplay.
'required by' says the answer is hidden in the clue (the answer is required to spell part of the clue).
'LYRE' is hidden within 'simply red'.
(Other definitions for lyre that I've seen before include "Ancient musical instrument - sounds mendacious" , "Music-maker" , "Instrument played by Orpheus" , "Old stringed instrument" , "Rely (anag.)" .)