Writing solution at side of book by right margin (5)
I believe the answer is:
brink
'margin' is the definition.
(thesaurus)
'writing solution at side of book by right' is the wordplay.
I cannot really understand how this works, but
'writing' could be 'r' (one of the traditional three R's of education) and 'r' is found within the answer.
'at' could be 'in' (synonymous in some cases - eg at school, in school) and 'in' is located in the answer.
'book' could be 'bk' (abbreviation for book) and 'bk' is present in the leftover letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for brink that I've seen before include "Threshold" , "Edge of precipice" , "Verge (of war etc)" , "Edge (of disaster etc)" , "Verge of an event" .)