Crossword Genius

A group in part of autumn commercial (5)

Ross

I believe the answer is:

octad

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

'a group in' is the definition.
The definition and answer can be both quantities as well as being singular nouns.
Perhaps you can see a link between them that I can't see?

'part of autumn commercial' is the wordplay.
'part of autumn' becomes 'oct' ('Oct' is part of 'autumn').
'commercial' becomes 'ad' (abbreviation for advertisement).
'oct'+'ad'='OCTAD'

Can you help me to learn more?

(Other definitions for octad that I've seen before include "Group" , "Crew, perhaps" , "E.g. quarterfinalists" , "quarter-finalists, perhaps" .)

I've seen this clue in The Independent.
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