Textbook three, maybe third of three (6)
I believe the answer is:
primer
'textbook' is the definition.
(I know that primer is a type of textbook)
'three maybe third of three' is the wordplay.
I cannot really see how this works, but
'maybe' could be 'i' (I've seen this in other clues) and 'i' is found within the answer.
'third of three' could be 'r' (3rd letter of 'three') and 'r' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for primer that I've seen before include "School textbook" , "introductory book" , "explosive device" , "Solitary" , "Initial coat of paint" .)