Musical direction, it being reversed by old king (5)
I believe the answer is:
tutti
'musical direction' is the definition.
(I've seen this in another clue)
'it being reversed by old king' is the wordplay.
'being reversed' says the letters should be written backwards.
'by' says to put letters next to each other.
'old king' becomes 'tut' (King Tut).
'it' reversed gives 'ti'.
'ti' put after 'tut' is 'TUTTI'.
(Other definitions for tutti that I've seen before include "Orchestral passage" , "Every instrument to play" , "with instruments and singers together" , "musical direction" , "All (stage direction)" .)