Cook claims that is hot (5)
I believe the answer is:
fiery
'hot' is the definition.
('fiery' can be a synonym of 'hot')
'cook claims that is' is the wordplay.
'cook' becomes 'fry' (I've seen this before).
'claims' indicates putting letters inside.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'fry' going around 'ie' is 'FIERY'.
(Other definitions for fiery that I've seen before include "Impetuous; hot-tasting" , "Explosive (e.g. temper)" , "Ardent" , "Intensely hot" , "Hot-tempered, passionate" .)