A nun ordered a large yearbook (6)
I believe the answer is:
Here is my best explanation:
'yearbook' is the definition.
('annual' can be a synonym of 'yearbook')
'a nun ordered a large' is the subsidiary indication.
'ordered' is an anagram indicator.
'large' becomes 'l'.
'nun' anagrammed gives 'nnu'.
(Other definitions for annual that I've seen before include "Occurring once every year", "Yearly book", "Plant that lives and dies within one year", "Every twelve months", "Book published every year", "Published yearly", "yearly event", "Regular publication", "Hardy it may be" and "Occuring once a year".)