Rooks circling are not so commonly seen (5)

I believe the answer is:

rarer

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'rooks circling are not so commonly seen' is the definition.
I can't tell whether this definition defines the answer.

'rooks circling are' is the wordplay.
'rooks' becomes 'RR' (chess abbreviation for rook written twice).
'circling' is an insertion indicator (circling can mean going around or containing).
'rr' enclosing 'are' is 'RARER'.

Can you help me to learn more?

(Other definitions for rarer that I've seen before include "Stranger" , "even less common" , "less easy to find" , "Less frequent" , "Thinner" .)

I've seen this clue in The Independent.

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