Old king rejected communist's command (5)
I believe the answer is:
order
'command' is the definition.
(order is a kind of command)
'old king rejected communist's' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'king' becomes 'r' (r is abbreviation for rex).
'rejected' shows that the letters should be reversed in order.
'communist' becomes 'red' (red is a kind of communist).
'red' backwards is 'der'.
'o'+'r'+'der'='ORDER'
(Other definitions for order that I've seen before include "Command; neatness" , "kind > direction" , "Class - instruction - book" , "Kind - sort" , "Regulation" .)