Old gathering by word of mouth (6)
I believe the answer is:
orally
'by word of mouth' is the definition.
(I've seen this in another clue)
'old gathering' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'gathering' becomes 'rally' (rally is a kind of gathering).
'o'+'rally'='ORALLY'
(Other definitions for orally that I've seen before include "given drugs thus?" , "via the trap" , "By spoken word" , "By (word of) mouth" , "How anyone would speak" .)