Main subject taken by students (12)

I believe the answer is:

oceanography

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

'main subject' is the definition.
'oceanography' can be an answer for 'subject' (oceanography is a kind of subject). I'm unsure of the 'main' bit.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for oceanography that I've seen before include "Scientific study of the seas" , "Study of the sea" .)

I've seen this clue in The Globe and Mail.

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