Official warning cowardly joker, say (6,4)
I believe the answer is:
yellow card
'official warning' is the definition.
(warning from an official in sports)
'cowardly joker say' is the wordplay.
'cowardly' becomes 'yellow' (similar in meaning).
'joker say' becomes 'card' (joker is a type of playing card).
'yellow'+'card'='YELLOW CARD'
(Other definitions for yellow card that I've seen before include "Footballer's caution" , "Warning leading to temporary dismissal" , "With which a referee cautions player" , "Warning to footballers" , "Warning to footballer" .)