The most common and most likely answer to this crossword clue is the 5 letter word WARDS
I've seen this crossword clue in The Guardian
W
A
R
D
S
More clues leading to the result WARDS
Buddy explains!

The straight part
We know that "cell blocks" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cell blocks = wards.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 WARDS mean?
WARDS noun- A warden; a guard; a guardian or watchman.
- Protection, defence.
- A protected place, and by extension, a type of subdivision.
- A person under guardianship.
- An object used for guarding.
- To keep in safety, to watch over, to guard.
- To defend, to protect.
- To fend off, to repel, to turn aside, as anything mischievous that approaches; -- usually followed by off.
- To be vigilant; to keep guard.
- To act on the defensive with a weapon.