Officer commanding little group (5)
I believe the answer is:
octad
'group' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'officer commanding little' is the wordplay.
'officer commanding' becomes 'oc' (this could be a standard abbreviation I've not previously seen).
'little' becomes 'tad' (both can mean a small amount).
'oc'+'tad'='OCTAD'
Can you help me to learn more?
(Other definitions for octad that I've seen before include "quarter-finalists, perhaps" , "Group" , "Crew, perhaps" , "E.g. quarterfinalists" .)