In Bangladesh a happy ruler (4)
I believe the answer is:
shah
'ruler' is the definition.
(I know that Shah is a type of crowned head)
'in bangladesh a happy' is the wordplay.
'in' indicates a hidden word.
'SHAH' is hidden in the letters of 'bangladesh a happy'.
(Other definitions for shah that I've seen before include "Title of Iranian monarch" , "Sovereign ousted in 1979" , "Title of deposed Iranian leader" , "Former Persian title" , "Former Persian ruler" .)