Ring's comprising principally rare jewels (6)
I believe the answer is:
pearls
'jewels' is the definition.
(pearl is a kind of jewel)
'ring's comprising principally rare' is the wordplay.
I cannot quite understand how this works, but
'rare' could be 'r' (listed by Oxford Dictionary of Abbreviations) and 'r' is present in the answer.
'ring' could be 'peal' ('peal' can be a synonym of 'ring') and 'peal' is present in the remaining letters.
A single letter 's' remains which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for pearls that I've seen before include "They're found in the shells of oysters" , "Nacreous gems" , "Gems from oysters" , "Precious particles in shellfish" , "Oyster gems" .)