Ten volunteers aren’t much (4)
I believe the answer is:
iota
'aren't much' is the definition.
The definition and answer can be both quantities as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'ten volunteers' is the wordplay.
'ten' becomes 'io'.
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'io'+'ta'='IOTA'
Can you help me to learn more?
(Other definitions for iota that I've seen before include "Tiny or scarcely detectable amount" , "Greek letter - small quantity" , "Grain" , "'Very small amount, jot (4)'" , "Foreign character" .)