A great number, say, eaten by cat (6)
I believe the answer is:
legion
'a great number' is the definition.
The definition and answer can be both groups as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'say eaten by cat' is the wordplay.
'say' becomes 'eg' (both can mean for example).
'eaten by' is an insertion indicator (inserted letters get eaten).
'cat' becomes 'lion' (lion is a kind of cat).
'eg' put within 'lion' is 'LEGION'.
Can you help me to learn more?
(Other definitions for legion that I've seen before include "A great number in the Roman military unit" , "Roman army grouping" , "Roman military formation" , "Many soldiers" , "Vast multitude" .)