Gained a single, say (3)
I believe the answer is:
won
'gained' is the definition.
(I've seen this before)
'a single say' is the wordplay.
'a single' becomes 'one' (a single thing - only one).
'say' shows a homophone (sound like).
'one' is a homophone of 'WON'.
(Other definitions for won that I've seen before include "Eastern currency" , "prevailed" , "Succeeded, conquered" , "little cash in Korea" , "Gained a victory" .)