One on hand, if learning is disrupted (10)
I believe the answer is:
fingernail
'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" .)