Number Two's tax disc sent in (6)
I believe the answer is:
deputy
'number two's' is the definition.
(both can mean a second-in-command or assistant)
'tax disc sent in' is the wordplay.
'tax' becomes 'duty' (duty is a kind of tax).
'disc' becomes 'EP' (extended play record).
'sent in' means one lot of letters goes inside another.
'duty' placed around 'ep' is 'DEPUTY'.
(Other definitions for deputy that I've seen before include "Second-in-command" , "Second in command" , "Person appointed to substitute for another" , "Substitute eg for sheriff" , "One who stands in" .)