Annexe about to run into chalk ground (7)
I believe the answer is:
whiting
'chalk ground' is the definition.
(whiting can mean ground chalk)
'annexe about to run into' is the wordplay.
'annexe' becomes 'wing' (synonyms).
'about' indicates putting letters inside.
'to run into' becomes 'hit' (eg hitting or running into with a car).
'wing' placed around 'hit' is 'WHITING'.
(Other definitions for whiting that I've seen before include "Small fish related to cod" , "[SOMETHING FISHY]" , "North Atlantic food fish" , "Edible marine fish" , "Small sea fish or ground chalk for painting" .)