Group once fashionable — a little (6)
I believe the answer is:
heptad
'group' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'fashionable a little' is the wordplay.
'fashionable' becomes 'hep' (I can't justify this - if you can you should believe this answer much more).
'a little' becomes 'tad' (both can mean a small amount).
'hep'+'tad'='HEPTAD'
'once' acts as a link.
Can you help me to learn more?
(Other definitions for heptad that I've seen before include "Set of seven" , "Seven Samurai maybe" , "one over the eight? Not quite that many!" , "A seven-strong group" , "Odd grouping" .)