Bar writer pens two fifths of story (6)
I believe the answer is:
bistro
'bar' is the definition.
(I have seen 'French bar' mean 'bistro' so perhaps 'bar' could also mean 'bistro')
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for bistro that I've seen before include "Eating place" , "Small informal French restaurant" , "Coming out of restaurant" , "Small inexpensive eatery" , "Small restaurant - robs it (anag)" .)