Hot tureen needs stir in addition to that (9)
I believe the answer is:
thereunto
'in addition to that' is the definition.
'thereunto' can be an answer for 'in addition' (I've seen this before). I am not certain of the rest of the definition.
'hot tureen needs stir' is the wordplay.
'needs stir' indicates an anagram (I've seen 'need stirring' mean this (I've seen this in other clues)).
'hot'+'tureen'='hottureen'
'hottureen' is an anagram of 'THEREUNTO'.
Can you help me to learn more?
(Other definitions for thereunto that I've seen before include "as well" , "to that place" , "To that (archaic)" , "Almost obsolete word meaning 'to that'; formally, 'in addition'" , "To that (formal)" .)