The most common and most likely answer to this crossword clue is the 5 letter word APISH
I've seen this crossword clue in The Times
A
P
I
S
H
More clues leading to the result APISH
Buddy explains!

The straight part
We know that "silly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case silly = apish.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 APISH mean?
APISH adjective- Resembling or characteristic of an ape