Not a levy on vice, but a grammatical consideration (6)
I believe the answer is:
syntax
'vice but a grammatical consideration' is the definition.
I can't tell whether this defines the answer.
'not a levy on' is the wordplay.
'not a' becomes 'syn' (I can't explain this - if you can you should believe this answer much more).
'levy' becomes 'tax' (taxing is a kind of levying).
'on' says to put letters next to each other.
'syn'+'tax'='SYNTAX'
Can you help me to learn more?
(Other definitions for syntax that I've seen before include "Linguistic study" , "Verbal order" , "Grammatical construction" , "Grammatical word structure in sentences" , "Sentence structure" .)