The most common and most likely answer to this crossword clue is the 10 letter word TRILATERAL
I've seen this crossword clue in The Times
T
R
I
L
A
T
E
R
A
L
Buddy explains!

The straight part
We know that "Having several sides" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case having several sides = trilateral.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 TRILATERAL mean?
TRILATERAL adjective- Having three sides
- Involving three parties