Nitwit turned on language. Thickening! (3,6)
I believe the answer is:
gum arabic
'thickening' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'nitwit turned on language' is the wordplay.
'nitwit' becomes 'mug' (I can't explain this - if you can you should believe this answer much more).
'turned' shows that the letters should be reversed in order.
'on' is a charade indicator (letters next to each other).
'language' becomes 'Arabic' (Arabic is a language).
'mug' in reverse letter order is 'gum'.
'gum'+'arabic'='GUM ARABIC'
Can you help me to learn more?
(Other definitions for gum arabic that I've seen before include "glue" , "Ingredient of 16 across" , "Exudation used in glues and incense" , "Substance exuded by acacia" .)