Blue tin with unknown content (4)

I believe the answer is:

cyan

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'blue' is the definition.
(I know that cyan is a type of blue)

'tin with unknown content' is the wordplay.
'tin' becomes 'can' (synonyms).
'with unknown' becomes 'y' (algebra. I am not sure about the 'with' bit.).
'content' indicates putting letters inside.
'can' placed around 'y' is 'CYAN'.

Can you help me to learn more?

(Other definitions for cyan that I've seen before include "Bluish-green" , "Bluish green" , "Blue printing ink" , "Greeny blue" , "Primary subtractive colour" .)

I've seen this clue in The Sun.

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