Physicist, heating element up, runs (4)
I believe the answer is:
bohr
'physicist' is the definition.
(I've seen this before)
'heating element up runs' is the wordplay.
'heating element' becomes 'hob' (as in a hob on a stove).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'runs' becomes 'r' (cricket abbreviation).
'hob' reversed gives 'boh'.
'boh'+'r'='BOHR'
(Other definitions for bohr that I've seen before include "Danish Nobel-winning physicist (father or son)" , "scientist" , "Niels -, Danish Nobel physicist" .)