The most common and most likely answer to this crossword clue is the 8 letter word PLANNING
I've seen this crossword clue in The Daily Telegraph
P
L
A
N
N
I
N
G
More clues leading to the result PLANNING
Buddy explains!

The straight part
We know that "Town hall department" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case town hall department = planning.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 PLANNING mean?
PLANNING verb- To design (a building, machine, etc.).
- To create a plan for.
- To intend.
- To make a plan.
- Action of the verb to plan.
- The act of formulating of a course of action, or of drawing up plans.
- The act of making contingency plans.
- Planning permission.