Supplication of one on his knees (6)
I believe the answer is:
prayer
'supplication' is the definition.
(I know that supplication can be written as prayer)
'one on his knees' is the wordplay.
I cannot really understand how this works, but
'one' could be 'a' (a thing is one thing) and 'a' is present in the answer.
The remaining letters 'pryer' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'of' is the link.
Can you help me to learn more?
(Other definitions for prayer that I've seen before include "Address to god" , "Earnest request" , "Supplication ot God" , "Collect - petition" , "Act of supplication to a deity" .)