Six in cover very angry (5)
I believe the answer is:
livid
'very angry' is the definition.
(I've seen this in another clue)
'six in cover' is the wordplay.
'six' becomes 'VI' (Roman numerals).
'in' is an insertion indicator.
'cover' becomes 'lid' (lid is a kind of cover).
'vi' inserted within 'lid' is 'LIVID'.
(Other definitions for livid that I've seen before include "... annoyed" , "Fuming" , "Apoplectic" , "Angry; marked with bruise" , "Very cross" .)