World's most popular writer? (4)
I believe the answer is:
biro
'writer?' is the definition.
(type of pen, something which writes)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for biro that I've seen before include "Lazlo -, ballpoint pen inventor" , "Pocket item" , "Laszlo Jozsef -, ballpoint pen inventor" , "Type of ballpoint pen" , "Writing implement" .)