Timid character that whistle-blower shows every so often (6,4)
I believe the answer is:
yellow card
'that whistle-blower shows every so often' is the definition.
(as in a referee - official with a whistle)
'timid character' is the wordplay.
'timid' becomes 'yellow' (yellow can mean cowardly).
'character' becomes 'card' (I've seen this before).
'yellow'+'card'='YELLOW CARD'
(Other definitions for yellow card that I've seen before include "With which a referee cautions player" , "Warning leading to temporary dismissal" , "Warning to footballer" , "Warning to footballers" , "Footballer's caution" .)