The most common and most likely answer to this crossword clue is the 8 letter word CONFINES
C
O
N
F
I
N
E
S
More clues leading to the result CONFINES
Buddy explains!

The straight part
We know that "pens" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pens = confines.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 CONFINES mean?
CONFINES verb- To restrict; to keep within bounds; to shut or keep in a limited space or area.
- To have a common boundary; to border; to lie contiguous; to touch; followed by on or with.
- The borders or limits of an area.
- Elements that restrain someone.
- The scope or range of a subject.