A book-end, for example, has a containing function (3)
I believe the answer is:
keg
'a containing function' is the definition.
I can't judge whether this definition defines the answer.
'a book-end for example' is the wordplay.
'end' indicates one should take the final letters.
'for example' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
The last letter of 'book' is 'k'.
'k'+'eg'='KEG'
'has' is the link.
Can you help me to learn more?
(Other definitions for keg that I've seen before include "Barrel as for beer" , "Sort of beer" , "Small beer cask" , "What's found at King's Head" , "Small cask or barrel" .)