Possibly writing a note on part of table (5)
I believe the answer is:
radon
'table' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'possibly writing a note on' is the wordplay.
'possibly' becomes 'on' (I can't explain this - if you can you should believe this answer much more).
'writing' becomes 'r' (one of the traditional three R's of education).
'note' becomes 'D' (musical note).
'on' is a charade indicator (letters next to each other).
'r'+'a'+'d'='rad'
'on' after 'rad' is 'RADON'.
'part of' is the link.
Can you help me to learn more?
(Other definitions for radon that I've seen before include "Radioactive gas, linked to granite" , "Gaseous element, Rn" , "Inert gas - adorn (anag)" , "gets no reaction" , "hazardous substance" .)