America on bottom row (6)
I believe the answer is:
rumpus
'row' is the definition.
(thesaurus)
'america on bottom' is the wordplay.
'america' becomes 'us' (abbreviation for the United States).
'on' means one lot of letters go next to another.
'bottom' becomes 'rump' ('rump' can be a synonym of 'bottom'**).
'us' put after 'rump' is 'RUMPUS'.
(Other definitions for rumpus that I've seen before include "Noisy brawl" , "Uproar or angry dispute" , "Row" , "Noisy, disruptive commotion" , "Making a fuss" .)