This heat is clearly defined (8)
I believe the answer is:
specific
I believe this clue is a double definition.
'this heat' is the first definition.
(specific heat is a kind of heat)
'clearly defined' is the second definition.
(I've seen this before)
'is' is the link.
(Other definitions for specific that I've seen before include "medieval drug" , "Clear and exact" , "Peculiar" , "Clearly defined or identified" , "remedy" .)