Colonies rash to support US cooperative gathering (8)
I believe the answer is:
beehives
'colonies' is the definition.
'rash to support us cooperative gathering' is the wordplay.
'rash' becomes 'hives' (hives is a kind of rash).
'to support' is a charade indicator (letters next to each other) (in a down clue, some letters hold others up).
'us cooperative gathering' becomes 'bee' (type of meeting for similar work eg a sewing bee).
'hives' put after 'bee' is 'BEEHIVES'.
(Other definitions for beehives that I've seen before include "An apiarist cares for them" , "Habitations for hymenoptera" .)