The most common and most likely answer to this crossword clue is the 6 plus 7 letter word PUBLIC SERVANT
I've seen this crossword clue in The Daily Telegraph
P
U
B
L
I
C
S
E
R
V
A
N
T
More clues leading to the result PUBLIC SERVANT
Buddy explains!
The straight part
We know that "one in the town hall" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one in the town hall = public servant.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.