Reduce the price when grade is blue (8)

I believe the answer is:

markdown

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'reduce the price' is the definition.
I can't judge whether this definition defines the answer.

'grade is blue' is the wordplay.
'grade' becomes 'mark' (synonyms).
'is blue' becomes 'down' (both can mean sad).
'mark'+'down'='MARKDOWN'

'when' acts as a link.

Can you help me to learn more?

(Other definitions for markdown that I've seen before include "Reduce in price" , "Reduction in price" , "Lower prices for 21" .)

I've seen this clue in The Irish Independent.

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