Daisy's teacher docking marks (5)

I believe the answer is:

aster

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

I can't explain the remainder of the clue.

(Other definitions for aster that I've seen before include "(Starlike) flower" , "Plant like a Michaelmas daisy" , "Christmas daisy" , "Plant of Michaelmas daisy family" , "Flower of the daisy family - rates (anag)" .)

I've seen this clue in The Sun.

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