It's normally measured at arm's length (6)
I believe the answer is:
sleeve
'arm's length' is the definition.
I know that 'arm' relates to this answer. I also know that both the answer and definition are singular nouns
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for sleeve that I've seen before include "Cover for a gramophone record" , "Covering for arm or record" , "Covering for arm or sleeve" , "Some wear their heart on it" , "Some wear their hearts on it" .)