The most common and most likely answer to this crossword clue is the 6 letter word BURIED
I've seen this crossword clue in Independent
B
U
R
I
E
D
Buddy explains!

The straight part
We know that "well-hidden" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case well-hidden = buried.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 BURIED mean?
BURIED adjective- Placed in a grave at a burial.
- Concealed, hidden.
- To ritualistically inter in a grave or tomb.
- To place in the ground.
- To hide or conceal as if by covering with earth or another substance.
- To suppress and hide away in one's mind.
- To put an end to; to abandon.
- To score a goal.
- To kill or murder.
- To render imperceptible by other, more prominent stimuli; drown out.