A physicist, he died after backing human rights (4)
I believe the answer is:
bohr
'a physicist' is the definition.
(I've seen this before)
'he died after backing human rights' is the wordplay.
'he died' becomes 'ob' ('ob.' can mean 'died' from the Latin obiit).
'after backing' says the letters should be written backwards.
'human rights' becomes 'hr' (this might be a standard abbreviation I've not previously seen).
'ob' backwards is 'bo'.
'bo'+'hr'='BOHR'
Can you help me to learn more?
(Other definitions for bohr that I've seen before include "Niels -, Danish Nobel physicist" , "Danish Nobel-winning physicist (father or son)" , "scientist" .)