American's money stolen -- this is taken by a gunman (8)
I believe the answer is:
assassin
'gunman' is the definition.
(thesaurus)
'american's money stolen this is taken by a' is the wordplay.
'american's' becomes 'a's' (common abbreviation - e.g. in organisation names).
'money' becomes 'sssi' (I can't explain this - if you can you should give a lot more credence to this answer).
'stolen' is an insertion indicator.
'this is taken by' means one lot of letters goes inside another.
'a' becomes 'an'.
'as' inserted inside 'sssi' is 'ssassi'.
'ssassi' going within 'an' is 'ASSASSIN'.
Can you help me to learn more?
(Other definitions for assassin that I've seen before include "One making hits" , "Killer of prominent figure" , "Liquidator" , "Killer of prominent person" , "I kill" .)