Crossword Genius

Very cross, if cover lets four or six through? (5)

Ross

I believe the answer is:

livid

I'm a little stuck... Click here to teach me more about this clue!

'very cross' is the definition.
(I've seen this before)

'cover lets four or six through?' is the wordplay.
I cannot really see how this works, but
'four' could be 'iv' (Roman numerals for four) and 'iv' is present in the answer.
'cover' could be 'lid' (lid is a kind of cover) and 'lid' is present in the remaining letters.
This accounts for all the letters.
This explanation may well be incorrect...

'if' is the link.

Can you help me to learn more?

(Other definitions for livid that I've seen before include "... annoyed" , "Black-and-blue - furious" , "Unpleasantly colourful" , "Furious (informal)" , "Enraged" .)

I've seen this clue in The Independent.
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