In the boom, a happy city on the Missouri (5)
I believe the answer is:
omaha
'city on the missouri' is the definition.
(I've seen this in another clue)
'in the boom a happy' is the wordplay.
'in' indicates a hidden word.
'OMAHA' is hidden in the letters of 'boom a happy'.
(Other definitions for omaha that I've seen before include "American-Indians" , "Where American may work" , "US city; D-Day 16" , "Cotton fabric used for curtains and upholstery" , "US 1944 D-Day landing beach" .)