The most common and most likely answer to this crossword clue is the 7 letter word STINKER
I've seen this crossword clue in Financial Times
S
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More clues leading to the result STINKER
- Second fiddle is one making a hum
- Scoundrel - rotten egg, perhaps?
- Very difficult question from small mischievous child
- Difficult task finally defeats metalworker
- Small mischevious child is a horror
- Problem for old Bob the repair man
- Predicament of a hummer
- Second fiddle is no easy thing
- Second fiddle? That’s a difficult question
- A problem that’s difficult as second fiddle
Buddy explains!

The straight part
We know that "Very difficult question" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case very difficult question = stinker.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 STINKER mean?
STINKER noun- A person who stinks.
- A contemptible person.
- Something difficult (e.g. a given puzzle) or unpleasant (e.g. negative review, nasty letter).
- Something of poor quality.
- Any of several species of large Antarctic petrels which feed on blubber and carrion and have an offensive odour, such as the giant fulmar.
- A chemist.
- A hot day.