The most common and most likely answer to this crossword clue is the 3 letter word KEN
I've seen this crossword clue in The Sunday Telegraph
K
E
N
Buddy explains!

The straight part
We know that "Understanding/chap" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case understanding/chap = ken.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 KEN mean?
KEN verb- To give birth, conceive, beget, be born; to develop (as a fetus); to nourish, sustain (as life).
- Knowledge, perception, or sight.
- Range of sight.
- To know, perceive or understand.
- To discover by sight; to catch sight of; to descry.
- (thieves' cant) A house, especially a den of thieves.
- A Japanese unit of length equal to six shakus