The most common and most likely answer to this crossword clue is the 6 letter word TAXING
I've seen this crossword clue in The Daily Telegraph
T
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Buddy explains!

The straight part
We know that "onerous" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case onerous = taxing.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 TAXING mean?
TAXING verb- To impose and collect a tax from (a person or company).
- To impose and collect a tax on (something).
- To make excessive demands on.
- To accuse.
- To examine accounts in order to allow or disallow items.
- The act of imposing a tax.
- With respect to an experience: exhausting; draining.
- Burdensome, difficult.