Two runs are so long! (3-3)
I believe the answer is:
bye-bye
'are so long' is the definition.
(I've seen this before)
'two runs' is the wordplay.
'two runs' means two substitutions for 'run'.
'run' becomes 'bye' (type of cricket run).
'run' becomes 'bye' (type of cricket run).
'bye'+'bye'='BYE-BYE'
(Other definitions for bye-bye that I've seen before include "So long" , "See you" , "Informal repetitive farewell" , "Ta-ta" , "Cheerio" .)