Stop to pen a book (6)
I believe the answer is:
annual
'book' is the definition.
(annual is a kind of book)
'stop to pen a' is the wordplay.
I cannot quite see how this works, but
'a' could be 'an' and 'an' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for annual that I've seen before include "plant > book" , "Occurring once every year" , "Coming once a year" , "Once a year, perhaps a book" , "(Of plant) living for a year or less" .)