Thus this writer requires paper on occasion (9)
I believe the answer is:
sometimes
'on occasion' is the definition.
(I've seen this before)
'thus this writer requires paper' is the wordplay.
'thus' becomes 'so' ('so' can be a synonym of 'thus').
'this writer' becomes 'me' (the person who is writing these clues).
'requires' says to put letters next to each other.
'paper' becomes 'times' (The Times newspaper**).
'so'+'me'+'times'='SOMETIMES'
(Other definitions for sometimes that I've seen before include "Seem moist (anag.)" , "Occasionally available" , "On occasion" , "Not always" .)