Magma stirred by third letter from Greece (5)

I believe the answer is:

gamma

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

'letter from greece' is the definition.
(letter from the Greek alphabet)

'magma stirred by third' is the wordplay.
'magma' can be anagrammed to 'GAMMA'.
Though, I'm not sure how the anagram is indicated.

Can you help me to learn more?

(Other definitions for gamma that I've seen before include "Letter from Greece" , "Foreign character" , "Greek letter - type of rays" , "Letter after alpha, beta" , "Third class, Gk. letter" .)

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

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