Members of militia heading up republic (5)
I believe the answer is:
haiti
'republic' is the definition.
(I know that Haiti is a republic)
'members of militia heading up' is the wordplay.
'members of' indicates a hidden word (the letters are 'members' of the clue).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'itiah' is hidden in the letters of 'militia heading'.
'itiah' reversed gives 'HAITI'.
(Other definitions for haiti that I've seen before include "Caribbean republic" , "West Indies country associated with voodoo" , "Caribbean state" , "Caribbean country, capital Port-au-Prince" , "I hit a West Indian island" .)