Convey one medium section (6)
I believe the answer is:
impart
'convey' is the definition.
(I know that convey is a more specific form of impart)
'one medium section' is the wordplay.
'one' becomes 'i' (Roman numeral).
'medium' becomes 'm' (as in UK clothing sizes).
'section' becomes 'part' (synonyms).
'i'+'m'+'part'='IMPART'
(Other definitions for impart that I've seen before include "Pass on information" , "Communicate (information)" , "Tell, pass on" , "Divulge, inform" , "grant" .)