Grammar topic (5)
'grammar topic' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for usage that I've seen before include "Way of employing" , "Treatment, practice" , "Customary practice" , "Application" , "Particular meaning of a word" .)
(Other definitions for syntax that I've seen before include "Rules governing the use of words" , "Grammatical arrangement of words" , "Linguistic study" , "The rules governing the arranging of words" , "Rules for sentence construction" .)