Fail to make joke, say, for periodical (6)
I believe the answer is:
digest
'periodical' is the definition.
(digest is a kind of periodical)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for digest that I've seen before include "Periodical that summarises the news" , "publication" , "Gradually come to understand" , "Shorten" , "Absorb, in stomach or mind" .)