Nobel and his mixed run as a scientist (5,4)
I believe the answer is:
niels bohr
'a scientist' is the definition.
(I've seen this before)
'nobel and his mixed run' is the wordplay.
'and' says to put letters next to each other.
'mixed' indicates an anagram.
'run' becomes 'r' (cricket abbreviation).
'nobel'+'his'='nobelhis'
'nobelhis' anagrammed gives 'nielsboh'.
'nielsboh'+'r'='NIELS BOHR'
'as' is the link.
(Other definitions for niels bohr that I've seen before include "Atomic scientist" , "Danish physicist" , "14 [GREAT DANE]" .)