Not much of a grin, perhaps (5)
I believe the answer is:
grain
'not much' is the definition.
The definition and answer can be both quantities as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'a grin perhaps' is the wordplay.
'perhaps' indicates an anagram.
'a'+'grin'='agrin'
'agrin' with letters rearranged gives 'GRAIN'.
'of' is the link.
Can you help me to learn more?
(Other definitions for grain that I've seen before include "Small hard seed of corn etc" , "little salty bit" , "Cereal seed" , "Small hard particle, of rice perhaps" , "Silo contents" .)