A game included in auction? Usually, but not always (2,1,4)
I believe the answer is:
as a rule
'usually but not always' is the definition.
(I've seen this before)
'a game included in auction?' is the wordplay.
'game' becomes 'ru' (rugby union).
'included in' indicates putting letters inside.
'auction?' becomes 'sale' (I've seen this in other clues).
'ru' put into 'sale' is 'sarule'.
'a'+'sarule'='AS A RULE'
(Other definitions for as a rule that I've seen before include "Generally, normally" , "26 [NORMALLY]" , "Usually, in the normal course" , "Mainly" , "none straighter" .)