By word of mouth round mass meeting (6)
I believe the answer is:
orally
'by word of mouth' is the definition.
(I've seen this before)
'round mass meeting' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'mass meeting' becomes 'rally' ('rally' can be a synonym of 'mass meeting').
'o'+'rally'='ORALLY'
(Other definitions for orally that I've seen before include "How one speaks" , "given drugs thus?" , "given by mouth" , "By (word of) mouth" , "via the trap" .)