One Danish scientist upset about source of many diamonds (6)
I believe the answer is:
rhombi
'diamonds' is the definition.
('rhombi' is the plural of 'rhombus')
'one danish scientist upset about source of many' is the wordplay.
'one' becomes 'i' (Roman numeral).
'danish scientist' becomes 'Bohr' (Niels Bohr was a Danish physicist).
'upset' says the letters should be written backwards.
'about' means one lot of letters goes inside another.
'source of' indicates taking the first letters (the start - or source - of the word).
The initial letter of 'many' is 'm'.
'i'+'bohr'='ibohr'
'ibohr' backwards is 'rhobi'.
'rhobi' enclosing 'm' is 'RHOMBI'.
(Other definitions for rhombi that I've seen before include "Forms of parallelograms" , "diamonds" , "Diamond shapes (maths.)" , "Lozenge shapes" , "certain figures in mathematics" .)