Complaint about Iris Murdoch's last tart (9)
I believe the answer is:
arthritis
'complaint' is the definition.
(arthritis is a kind of health problem)
'about iris murdoch's last tart' is the wordplay.
'about' indicates an anagram.
'last' says to take the final letters.
The last letter of 'murdoch' is 'h'.
'iris'+'h'+'tart'='irishtart'
'irishtart' anagrammed gives 'ARTHRITIS'.
(Other definitions for arthritis that I've seen before include "Inflammatory disease of the joints" , "Inflammation of joints" , "complaint" , "Joint pain" .)