Stiffener produced by celebrity chef? Not half! (6)
I believe the answer is:
starch
'stiffener' is the definition.
(I've seen this before)
'by celebrity chef? not half' is the wordplay.
'by' means one lot of letters go next to another.
'celebrity' becomes 'star' (I've seen this before).
'not half' suggests halving the letters.
'chef' with half the letters taken is 'ch'.
'star' put next to 'ch' is 'STARCH'.
'produced' acts as a link.
(Other definitions for starch that I've seen before include "Carbohydrate that occurs in all green plants" , "Carbohydrate found in potatoes, wheat etc." , "Stiffening agent" , "Carbohydrate found in potatoes, wheat, and rice" , "ceremony" .)