Fiddling at wrist, chap may let one slip off (10)

'slip off' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's a link between them I don't understand?

'fiddling at wrist chap may let one' is the wordplay.
I cannot quite see how this works, but
'at' is found in the answer.
'one' could be 'a' (a thing is one thing) and 'a' is present in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?