Kneel on ground clasping back of limb, behind a lower body part (5,4)

I believe the answer is:

ankle bone

'lower body part' is the definition.
(part of the body)

'kneel on ground clasping back of limb behind a' is the wordplay.
I cannot quite understand how this works, but
'on' is found in the answer.
'a' could be 'an' and 'an' is present in the answer.
This explanation may well be incorrect...

(Another definition for ankle bone that I've seen is " Part of leg".)

I've seen this clue in The Times.

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