Two thousand just supporting company as a rule (8)
I believe the answer is:
commonly
'as a rule' is the definition.
(thesaurus)
'two thousand just supporting company' is the wordplay.
'two thousand' becomes 'mm'.
'just' becomes 'only' ('only' can be a synonym of 'just').
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'company' becomes 'co' (abbreviation).
'mm'+'only'='mmonly'
'mmonly' after 'co' is 'COMMONLY'.
(Other definitions for commonly that I've seen before include "Usually, normally" , "Usually, ordinarily" , "as a general rule" , "in a plebeian manner?" , "Not how the aristocracy behave" .)