The most common and most likely answer to this crossword clue is the 4 plus 7 letter word WORD SQUARES
W
O
R
D
S
Q
U
A
R
E
S
Buddy explains!
The straight part
We know that "simple filled grids" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case simple filled grids = word squares.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.