Develop case of gangrene, say (7)
I believe the answer is:
gestate
'develop' is the definition.
(I've seen this before)
'case of gangrene say' is the wordplay.
'case of' means to remove the middle letters (outsides of).
'say' becomes 'state' ('state' can be a synonym of 'say').
'gangrene' with its middle taken out is 'ge'.
'ge'+'state'='GESTATE'
(Other definitions for gestate that I've seen before include "Develop over time" , "Evolve, ripen" , "Develop slowly" , "Develop (idea, foetus)" , "Carry in the womb" .)