Baking a gluten tart initially is a challenge (8)
I believe the answer is:
gauntlet
'a challenge' is the definition.
(to take up the gauntlet is to accept a challenge)
'baking a gluten tart initially' is the wordplay.
'baking' indicates an anagram (letters baked or cooked into a new form).
'initially' indicates taking the first letters.
The initial letter of 'tart' is 't'.
'a'+'gluten'='agluten'
'agluten' anagrammed gives 'gauntle'.
'gauntle'+'t'='GAUNTLET'
'is' acts as a link.
(Other definitions for gauntlet that I've seen before include "Long-sleeved glove" , "that may be punishing for runner" , "Knight's glove" , "Strong glove, thrown down in challenge" , "Throw it down to offer a challenge" .)