The most common and most likely answer to this crossword clue is the 13 letter word COUNTERWEIGHT
I've seen this crossword clue in The Sunday Telegraph
C
O
U
N
T
E
R
W
E
I
G
H
T
Buddy explains!
The straight part
We know that "balance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case balance = counterweight.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 COUNTERWEIGHT mean?
COUNTERWEIGHT noun- A heavy mass of often iron or concrete, mechanically linked in opposition to a load which is to be raised and lowered, with the intent of reducing the amount of work which must be done to effect the raising and lowering. Counterweights are used, for example, in cable-hauled elevators and some kinds of movable bridges (e.g. a bascule bridge).
- To fit with a counterweight.