Gathers American is supported by fools (7)
I believe the answer is:
amasses
'gathers' is the definition.
('amass' can be a synonym of 'gather')
'american is supported by fools' is the wordplay.
'american' becomes 'Am' (abbreviation in some contexts).
'is supported by' says to put letters next to each other (in a down clue, some letters hold up others).
'fools' becomes 'asses' (ass can mean a foolish person).
'am'+'asses'='AMASSES'
(Other definitions for amasses that I've seen before include "Gathers together" , "'Accumulates, gathers (7)'" , "Collects" .)