Dummy American on swamp (5)
I believe the answer is:
bogus
'dummy' is the definition.
(I've seen this before)
'american on swamp' is the wordplay.
'american' becomes 'us'.
'on' says to put letters next to each other.
'swamp' becomes 'bog' (I've seen this before).
'us' put after 'bog' is 'BOGUS'.
(Other definitions for bogus that I've seen before include "Spurious" , "Not true" , "that's not real" , "Dummy" , "Not genuine, sham" .)