What was for writing in quiz, mostly fifty-fifty (5)
I believe the answer is:
quill
'what was for writing' is the definition.
(quills were historically used to write)
'in quiz mostly fifty-fifty' is the wordplay.
'in' is an insertion indicator.
'mostly' means to remove the last letter.
'fifty' becomes 'L' (Roman numeral).
'fifty' becomes 'L' (Roman numeral).
'quiz' with its last letter taken off is 'qui'.
'qui'+'l'='quil'
'quil' placed around 'l' is 'QUILL'.
(Other definitions for quill that I've seen before include "Porcupine spine" , "Old writer" , "Feather for writing" , "Old pen" , "Pen made from a feather" .)