Spurious marsh seen by American (5)
I believe the answer is:
bogus
'spurious' is the definition.
(I've seen this in another clue)
'marsh seen by american' is the wordplay.
'marsh' becomes 'bog' (a bog is a marshy area).
'seen by' says to put letters next to each other.
'american' becomes 'US' (United States).
'bog'+'us'='BOGUS'
(Other definitions for bogus that I've seen before include "Dummy" , "that's not real" , "Scribbled drawing" , "Not genuine, sham" , "Not true" .)