By mentioning a purchase (3)
I believe the answer is:
buy
'a purchase' is the definition.
(buy is a kind of purchase)
'by mentioning' is the wordplay.
'mentioning' shows a homophone (sound like) (I've seen 'mentioned' mean this).
'by' is a homophone of 'BUY'.
(Other definitions for buy that I've seen before include "Purchase" , "Accept truth of (colloq.)" , "Get" , "Procure" .)