Colours textbook (8)
I believe the answer is:
standard
This clue is a double definition.
'colours' is the first definition.
(I've seen this before)
'textbook' is the second definition.
(thesaurus)
(Other definitions for standard that I've seen before include "sort of lamp" , "thing seen looking down The Mall?" , "Flag - of the usual kind" , "Level of quality; flag" , "Normal - colours" .)