Mainly like beer, covering game (2,1,4)
I believe the answer is:
as a rule
Here is my best explanation:
'mainly' is the definition.
'like beer covering game' is the subsidiary indication.
'like' becomes 'as'.
'beer' becomes 'ale' (I've seen this before).
'covering' means one lot of letters goes inside another.
'game' becomes 'ru'.
'ale' placed around 'ru' is 'arule'.
(Other definitions for as a rule that I've seen before include "none straighter", "In most cases", "Mainly", "Usually, in general", "in the main", "Usually, in the normal course", "In general", "Normally", "Most of the time" and "generally".)