Swelling with unlimited nitrogen fixation (6)

I believe the answer is:

goitre

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

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

I don't understand the rest of the clue.

(Other definitions for goitre that I've seen before include "Abnormally enlarged thyroid gland" , "Enlarged thyroid gland with swollen neck" , "Enlarged thyroid gland showing as neck swelling" .)

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

Want a hint initially instead of a full solution? Install my app