The most common and most likely answer to this crossword clue is the 4 letter word DEEP
I've seen this crossword clue in The Guardian
D
E
E
P
More clues leading to the result DEEP
- Bottomless river associated with Parana's source
- Well down in a place where water is plentiful
- Mysterious river ending in swamp
- Hard to fathom?
- Extending far underground, weed comes back
- Produced a tinkle, inverted and low
- Cunning, in the main
- Went backwards to a great extent
- Went running back when late
- Leading elements of dissertation, extremely esoteric paper, hard to follow
Buddy explains!

The straight part
We know that "Cunning,/the main" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cunning,/the main = deep.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 DEEP mean?
DEEP noun- (with "the") The deep part of a lake, sea, etc.
- (with "the") A silent time; quiet isolation.
- A deep shade of colour.
- The profound part of a problem.
- (with "the") The sea, the ocean.
- A fielding position near the boundary.
- (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.
- Deeply.