Hidden in cover, a girl is really angry! (5)
I believe the answer is:
livid
'really angry' is the definition.
(I've seen this in another clue)
'hidden in cover a girl' is the wordplay.
'hidden in' means one lot of letters goes inside another.
'cover' becomes 'lid' (lid is a kind of cover).
'a girl' becomes 'vi' (short for Violet).
'lid' enclosing 'vi' is 'LIVID'.
'is' acts as a link.
(Other definitions for livid that I've seen before include "Angry; marked with bruise" , "Black-and-blue - furious" , "Apoplectic" , "infuriated" , "Intensely angry - dark bluish grey" .)