Opened, revealing the mixture inside the pudding (5)
I believe the answer is:
undid
'opened' is the definition.
(I've seen this in another clue)
'the mixture inside the pudding' is the wordplay.
'the mixture' indicates an anagram.
'inside' means to look at the middle letters.
The centre of 'pudding' is 'uddin'.
'uddin' with letters rearranged gives 'UNDID'.
'revealing' acts as a link.
(Other definitions for undid that I've seen before include "solved knotty problem" , "Annulled; unfastened" , "Cancelled - destroyed" , "Disentangled" , "Loosened; ruined" .)