Danish physicist fixed NHS boiler (5,4)
I believe the answer is:
niels bohr
'danish physicist' is the definition.
(I've seen this before)
'fixed nhs boiler' is the wordplay.
'fixed' indicates anagramming the letters.
'nhs'+'boiler'='nhsboiler'
'nhsboiler' with letters rearranged gives 'NIELS BOHR'.
(Other definitions for niels bohr that I've seen before include "Atomic scientist" , "Danish physicist" , "14 [GREAT DANE]" .)