The most common and most likely answer to this crossword clue is the 6 letter word ONCOST
O
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More clues leading to the result ONCOST
Buddy explains!

The straight part
We know that "overhead" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case overhead = oncost.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 ONCOST mean?
ONCOST noun- Additional costs; extra expenses.
- Overhead (expense)
- (coal-mining) The cost of upkeep, making and maintaining shafts, roads, pumping, etc.
- The indirect expenditure incurred for the purpose of increasing the productive power of organised labour.