Obligatory paper in colour (8)
I believe the answer is:
required
'obligatory' is the definition.
(similar in meaning)
'paper in colour' is the wordplay.
'paper' becomes 'quire' (paper measurement).
'in' indicates putting letters inside.
'colour' becomes 'red' (red is a kind of colour).
'quire' inserted into 'red' is 'REQUIRED'.
(Other definitions for required that I've seen before include "Needed" , "Essential, compulsory" , "Not optional" .)