Mainly like beer, covering game (2,1,4)
I believe the answer is:
as a rule
'mainly' is the definition.
(I've seen this before)
'like beer covering game' is the wordplay.
'like' becomes 'as'.
'beer' becomes 'ale' (I've seen this before).
'covering' means one lot of letters goes inside another.
'game' becomes 'ru' (rugby union).
'ale' enclosing 'ru' is 'arule'.
'as'+'arule'='AS A RULE'
(Other definitions for as a rule that I've seen before include "Under normal circumstances" , "In most cases" , "26 [NORMALLY]" , "Generally, normally" , "Usually, in general" .)