Confectioner's pastries ruined diet, essentially (10)
I believe the answer is:
patisserie
'confectioner's' is the definition.
(patisserie is a type of confectioner)
'pastries ruined diet essentially' is the wordplay.
'ruined' indicates an anagram.
'essentially' says to take the centre.
The central letters of 'diet' are 'ie'.
'pastries' with letters rearranged gives 'patisser'.
'patisser'+'ie'='PATISSERIE'
(Other definitions for patisserie that I've seen before include "food shop" , "supplier of tarts" , "where cakes are sold" , "eclairs and the like" , "Its cakes are fancy" .)