In bloom, a happy US city (5)
I believe the answer is:
omaha
'us city' is the definition.
(I've seen this before)
'in bloom a happy' is the wordplay.
'in' indicates the answer is hidden within the clue.
'OMAHA' can be found hidden inside 'bloom a happy'.
(Other definitions for omaha that I've seen before include "State [CITY!!!]" , "Nebraska city and code-named beach of D-Day landings" , "This was old Indian territory" , "US 1944 D-Day landing beach" , "American Indian" .)