A group in part of autumn commercial (5)
I believe the answer is:
octad
'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" .)