Finishing touches for scripts by Dan Levy, showing correct use of language (6)
I believe the answer is:
syntax
'correct use of language' is the definition.
(grammar)
'finishing touches for scripts by dan levy' is the wordplay.
'finishing touches for' indicates one should take the final letters (the finishing letters).
'levy' becomes 'tax' (taxing is a kind of levying).
The last letters of 'scripts by dan' are 'syn'.
'syn'+'tax'='SYNTAX'
'showing' acts as a link.
(Other definitions for syntax that I've seen before include "Rules governing the use of words" , "Rules governing formation of sentences and phrases" , "Grammatical arrangement of words" , "The rules governing the arranging of words" , "Rules for sentence construction" .)