Pen memoirs about king (4)
I believe the answer is:
biro
'pen' is the definition.
(Biro is a make of pen)
'memoirs about king' is the wordplay.
'memoirs' becomes 'bio' (I can't justify this - if you can you should give a lot more credence to this answer).
'about' is an insertion indicator.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'bio' going around 'r' is 'BIRO'.
Can you help me to learn more?
(Other definitions for biro that I've seen before include "Type of ballpoint pen" , "Pocket item" , "Lazlo -, ballpoint pen inventor" , "writer" , "Laszlo Jozsef -, ballpoint pen inventor" .)