Bemoan somehow about onset of Delhi belly (7)
I believe the answer is:
abdomen
'belly' is the definition.
('abdomen' can be a synonym of 'belly')
'bemoan somehow about onset of delhi' is the wordplay.
'somehow' is an anagram indicator.
'about' indicates putting letters inside.
'onset of' indicates taking the first letters.
The first letter of 'delhi' is 'd'.
'bemoan' is an anagram of 'abomen'.
'abomen' enclosing 'd' is 'ABDOMEN'.
(Other definitions for abdomen that I've seen before include "Belly" , "Mean bod (anag.)" , "guts" , "Site of corporation" , "Where one digests" .)