Old music producer very likely showing up (4)
I believe the answer is:
lyre
'old music producer' is the definition.
(ancient instrument)
'very likely showing up' is the wordplay.
'showing' indicates a hidden word.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'eryl' is hidden in the letters of 'very likely'.
'eryl' backwards is 'LYRE'.
(Other definitions for lyre that I've seen before include "Instrument played by Orpheus" , "used to produce music" , "Ancient stringed musical instrument with U-shaped frame" , "Ancient harp-like instrument" , "Rely (anag.)" .)