Crossword Genius

One on hand, if learning is disrupted (10)

Ross

I believe the answer is:

fingernail

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'one on hand' is the definition.
The answer and definition can be both body parts as well as being singular nouns.
Maybe you can see a link between them that I can't see?

'if learning is disrupted' is the wordplay.
'is disrupted' is an anagram indicator.
'if'+'learning'='iflearning'
'iflearning' is an anagram of 'FINGERNAIL'.

Can you help me to learn more?

(Other definitions for fingernail that I've seen before include "One in ten" , "extreme protection results" , "Part of hand" .)

I've seen this clue in The Guardian.
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