The tar developed during cooking is overwhelming (12)
I believe the answer is:
breathtaking
'is overwhelming' is the definition.
(thesaurus)
'the tar developed during cooking' is the wordplay.
'developed' is an anagram indicator.
'during' indicates putting letters inside.
'cooking' becomes 'baking' (baking is a kind of cooking**).
'the'+'tar'='thetar'
'thetar' with letters rearranged gives 'reatht'.
'reatht' going within 'baking' is 'BREATHTAKING'.
(Other definitions for breathtaking that I've seen before include "Causing astonishment" , "Moving" , "Awe-inspiring" , "Astounding" , "Awesome" .)