Writer briefly pens a flyer (5)
I believe the answer is:
quail
'flyer' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'writer briefly pens a' is the wordplay.
'writer' becomes 'quill' (item which writes).
'briefly' means to remove the last letter.
'pens' is an insertion indicator (inserted letters are held or penned in).
'quill' with its final letter taken away is 'quil'.
'quil' going around 'a' is 'QUAIL'.
Can you help me to learn more?
(Other definitions for quail that I've seen before include "See little bird" , "Small bird of the partridge family" , "recoil with fear" , "Shrink; a bird" , "Game bird - cower" .)