Sentence incorporates clauses primarily as written (3)
I believe the answer is:
sic
'as written' is the definition.
(I've seen this in another clue)
'sentence incorporates clauses primarily' is the wordplay.
'primarily' indicates taking the first letters.
The first letters of 'sentence incorporates clauses' is 'SIC'.
(Other definitions for sic that I've seen before include "Printing term to denote accuracy though it appears otherwise" , "Thus in Corsica" , "Thus - in Latin" , "'Thus, in printing (3)'" , "(Of quoted word) exactly so" .)