Form master to divide students by ability (6)

I believe the answer is:

stream

I'm a little stuck... Click here to teach me more about this clue!

'form' is the definition.
(I know that stream is a Finnish National Land Survey place type)

'master to divide students by ability' is the wordplay.
'master' can be anagrammed to 'STREAM'.
Though, I am not sure how the anagram is indicated.

Can you help me to learn more?

(Other definitions for stream that I've seen before include "schoolchildren grouped together" , "group of pupils split by ability" , "comprising a load of drips?" , "Transmit music or video online" , "Quantity of water" .)

I've seen this clue in the Sydney Morning Herald.

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