It may be used to measure a diamond in a ring perhaps (5)
I believe the answer is:
grain
'it may be used to measure a diamond' is the definition.
The definition suggests a singular noun which matches the answer.
'a ring perhaps' is the wordplay.
'perhaps' is an anagram indicator.
'a'+'ring'='aring'
'aring' is an anagram of 'GRAIN'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for grain that I've seen before include "Enter into combat with" , "Cereal seed" , "Silo contents" , "Small weight; seed" , "Character" .)