Crossword Genius

Variation in same key will make it really dark (3,4)

Ross

I believe the answer is:

eye mask

Here is my best explanation:

'will make it really dark' is the definition.
(someone wearing an eye mask is in the dark)

'variation in same key' is the wordplay.
'variation in' indicates an anagram.
'same'+'key'='samekey'
'samekey' is an anagram of 'EYE MASK'.

This clue was last seen in The Guardian