Generally, a game may be found in auction (2,1,4)
I believe the answer is:
as a rule
'generally' is the definition.
(I've seen this in another clue)
'a game may be found in auction' is the wordplay.
'game' becomes 'RU' (rugby union).
'may be found in' indicates putting letters inside.
'auction' becomes 'sale' (I've seen this before).
'ru' going into 'sale' is 'sarule'.
'a'+'sarule'='AS A RULE'
(Other definitions for as a rule that I've seen before include "in the main" , "Usually, in general" , "none straighter" , "Generally, normally" , "Usually but not always" .)