Completely hot mixture - not exactly (10)
I believe the answer is:
thoroughly
'completely' is the definition.
(I've seen this before)
'hot mixture not exactly' is the wordplay.
'mixture' indicates an anagram.
'not exactly' becomes 'roughly' (rough can mean inexact or approximate).
'hot' is an anagram of 'tho'.
'tho'+'roughly'='THOROUGHLY'
(Other definitions for thoroughly that I've seen before include "Completely" , "Really" , "Quite" , "More than a bit" , "Musical with Julie Andrews" .)