The most common and most likely answer to this crossword clue is the 8 plus 6 letter word REPORTED SPEECH
R
E
P
O
R
T
E
D
S
P
E
E
C
H
Buddy explains!
The straight part
We know that "Grammatical device" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case grammatical device = reported speech.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.