Biochemist's first idea for treatment set back medical condition (8)
I believe the answer is:
diabetes
'medical condition' is the definition.
(I've seen this before)
'biochemist's first idea for treatment set back' is the wordplay.
'biochemist's first' becomes 'b' (1st letter of 'biochemist').
'for treatment' indicates an anagram (the anagram must be processed or treated).
'back' says the letters should be written in reverse.
'set' in reverse letter order is 'tes'.
'b'+'idea'='bidea'
'bidea' with letters rearranged gives 'diabe'.
'diabe'+'tes'='DIABETES'
(Other definitions for diabetes that I've seen before include "Insulin-related disorder" , "illness" , "Disorder causing excessive urination" , "Blood disorder" , "Effect of lack of insulin" .)