The most common and most likely answer to this crossword clue is the 9 plus 8 letter word MISPLACED MODIFIER
I've seen this crossword clue in The Times
M
I
S
P
L
A
C
E
D
M
O
D
I
F
I
E
R
Buddy explains!
The straight part
We know that "grammatical solecism" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case grammatical solecism = misplaced modifier.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.