Crossword Genius

Circle of light generators, first three are this type (7)

Ross

I believe the answer is:

halogen

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'this type' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'circle of light generators first three' is the wordplay.
'circle of light' becomes 'halo' (I've seen this before).
'generators first' becomes 'g' (est letter of 'generators').
'three' becomes 'en' (I can't explain this - if you can you should believe this answer much more).
'halo'+'g'+'en'='HALOGEN'

'are' is the link.

Can you help me to learn more?

(Other definitions for halogen that I've seen before include "iodine, perhaps" , "One of group of five elements (used in this type of lamp)" , "I must be regarded as one" , "Element such as iodine" , "I too maybe" .)

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