The most common and most likely answer to this crossword clue is the 7 letter word DAFTEST
I've seen this crossword clue in Independent
D
A
F
T
E
S
T
More clues leading to the result DAFTEST
Buddy explains!

The straight part
We know that "Extremely silly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case extremely silly = daftest.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.
What does DAFTEST mean?
DAFTEST adjective- Foolish, silly, stupid.
- Crazy, insane, mad.
- Gentle, meek, mild.