Most painful - well, put your feet up (6)
I believe the answer is:
sorest
'most painful' is the definition.
('sore' can be a synonym of 'painful')
'well put your feet up' is the wordplay.
I cannot quite see how this works, but
'well' could be 'so' (both can mean extremely) and 'so' is found within the answer.
'put' could be 'rest' (resting is a kind of putting) and 'rest' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sorest that I've seen before include "Most tender" , "In greatest discomfort" , "Most aggrieved" , "Stores in most painful way" , "Most wounded" .)