The most common and most likely answer to this crossword clue is the 7 letter word DEEPEST
I've seen this crossword clue in The Guardian
D
E
E
P
E
S
T
More clues leading to the result DEEPEST
- At its most low, river a nuisance
- Extremely rich Scottish banker — it’s annoying
- One side of capital city on river, most significant
- Scottish, Welsh, English or Irish flower blight -- which is most intense?
- Most mysterious bug seen by river
- Nuisance, having to shore up river that’s most low
- Flower with blight — that’s most serious
- Least shallow river — nuisance going under
- Most intense river disease
- Extremely intense training in river’s heading for trouble
Buddy explains!

The straight part
We know that "Extremely rich" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case extremely rich = deepest.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 DEEPEST mean?
DEEPEST adjective- (of a physical distance) Extending far away from a point of reference, especially downwards.
- (intellectual, social) Complex, involved.
- (sound, voice) Low in pitch.
- (of a color) Highly saturated.
- (sleep) Sound, heavy (describing a state of sleep from which one is not easily awoken).
- Immersed, submerged (in).
- Muddy; boggy; sandy; said of roads.
- (of time) Distant in the past, ancient.