Crossword Genius

It's unusual to obtain rare stamp (10)

Ross

I believe the answer is:

aberration

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'stamp' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

'it's unusual to obtain rare' is the wordplay.
'obtain rare' anagrams to 'ABERRATION'.
But, I'm unsure how the anagram is indicated.

Can you help me to learn more?

(Other definitions for aberration that I've seen before include "instability" , "Mental lapse" , "Slip" , "Wandering" , "Unwelcome deviation from normal" .)

I've seen this clue in the Sydney Morning Herald.
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