The most common and most likely answer to this crossword clue is the 6 letter word DEEPLY
I've seen this crossword clue in Financial Times
D
E
E
P
L
Y
More clues leading to the result DEEPLY
- Letter spelled out work in detail
- Yelped painfully and intensely
- Yelped when beaten — with a resonant voice?
- Profoundly deaf leaders’ answer topped
- Character husband needs in the end to work at with intensity
- Keenly make regular trips south of river
- Banker’s thickness displayed thoroughly
- Work under river – far down?
Buddy explains!

The straight part
We know that "far down?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case far down? = deeply.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 DEEPLY mean?
DEEPLY adverb- At depth, in a deep way.
- To a deep extent.
- Profoundly.