The most common and most likely answer to this crossword clue is the 12 letter word COUNTERPOISE
I've seen this crossword clue in Independent
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More clues leading to the result COUNTERPOISE
Buddy explains!

The straight part
We know that "Offset" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case offset = counterpoise.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 COUNTERPOISE mean?
COUNTERPOISE noun- A weight sufficient to balance another, for example in the opposite end of scales; an equal weight.
- An equal power or force acting in opposition; a force sufficient to balance another force.
- The relation of two weights or forces which balance each other; equilibrium
- To act against with equal weight; to equal in weight; to balance the weight of; to counterbalance.
- To act against with equal power; to balance.