A great number forming part of Caesar's force (6)
I believe the answer is:
legion
'force' is the definition.
(I know that legion is a type of force)
'a great number forming part of caesar's' is the wordplay.
I cannot really see how this works, but
'great' could be 'g' (abbreviation in place names) and 'g' is found in the answer.
'number' could be 'l' (L is an example) and 'l' is present in the answer.
'of' could be 'o' and 'o' is present in the answer.
'part' could be 'in' (synonyms) and 'in' is located in the remaining letters.
A single letter 'e' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for legion that I've seen before include "Large body of soldiers" , "Roman battalion" , "Large military force" , "Division of ancient Roman army" , "Countless" .)