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 there's a link between them I don't understand?
'a grin perhaps' is the wordplay.
'perhaps' indicates an anagram.
'a'+'grin'='agrin'
'agrin' anagrammed gives 'GRAIN'.
'of' acts as a 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" .)