Say a case of gangrene is normal (7)
I believe the answer is:
average
'normal' is the definition.
('average' can be a synonym of 'normal')
'say a case of gangrene' is the wordplay.
'say' becomes 'aver' ('aver' can be a synonym of 'say').
'case of' says to hollow out the word (remove centre letters) (outsides of).
'gangrene' with its middle taken out is 'ge'.
'aver'+'a'+'ge'='AVERAGE'
'is' is the link.
(Other definitions for average that I've seen before include "Fair to middling" , "Moderate" , "Mean - everyday" , "Middle value, the norm" , "Run of the mill" .)