Crossword Genius

Daisy's teacher taking marks off (5)

Ross

I believe the answer is:

aster

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'daisy's' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for aster that I've seen before include "Stare (anag.)" , "Tears (anag) - plant" , "It grows" , "Daisy-like plant" , "Michaelmas daisy genus" .)

I've seen this clue in The Independent.
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