Element of poster perhaps one's cutting off the wall (6)
I believe the answer is:
radium
'element' is the definition.
(radium is a kind of element)
'poster perhaps one's cutting off the wall' is the wordplay.
I cannot quite understand how this works, but
'poster' could be 'ad' (short for advert) and 'ad' is found within the answer.
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
The remaining letters 'rum' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'of' is the link.
Can you help me to learn more?
(Other definitions for radium that I've seen before include "Radioactive metal" , "Element with symbol Ra" , "Curie-found element" , "Metallic element discovered by the Curies" , "Ra (atomic number 88)" .)