Company that is two thousand in red (6)
I believe the answer is:
commie
'red' is the definition.
(thesaurus)
'company that is two thousand in' is the wordplay.
'company' becomes 'co' (abbreviation).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'two' indicates the doubling of some letters.
'thousand' becomes 'm' ('m' can be a synonym of 'thousand'**).
'in' indicates putting letters inside.
'm' duplicated is 'mm'.
'co'+'ie'='coie'
'coie' placed around 'mm' is 'COMMIE'.
(Other definitions for commie that I've seen before include "Extreme left-winger" , "Eg, Marxist (colloq.)" , "Red" , "Left wing" .)