Circle inside a stent is rather severe in manner (10)
I believe the answer is:
astringent
'rather severe in manner' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'circle inside a stent' is the wordplay.
'circle' becomes 'ring' (synonyms).
'inside' means one lot of letters goes inside another.
'a'+'stent'='astent'
'ring' put inside 'astent' is 'ASTRINGENT'.
'is' is the link.
Can you help me to learn more?
(Other definitions for astringent that I've seen before include "Styptic" , "acerbic" , "Scathing" , "description of wine?" , "(Of taste) sharp, bitter" .)