Caution shown when playing king, fearful scoundrel nicking it? (6,4)
I believe the answer is:
yellow card
'caution shown when playing' is the definition.
(shown to a footballer as a caution)
'king fearful scoundrel nicking it?' is the wordplay.
'king' becomes 'r' (r is abbreviation for rex).
'fearful' becomes 'yellow' (similar in meaning).
'scoundrel' becomes 'cad' (cad is a kind of scoundrel).
'nicking it?' indicates putting letters inside (some letters stolen or nicked by others).
'yellow'+'cad'='yellowcad'
'r' placed into 'yellowcad' is 'YELLOW CARD'.
(Other definitions for yellow card that I've seen before include "Footballer's caution" , "With which a referee cautions player" , "Warning to footballers" , "Warning leading to temporary dismissal" , "Warning to footballer" .)