The most common and most likely answer to this crossword clue is the 5 letter word CLIFF
I've seen this crossword clue in The Times
C
L
I
F
F
Buddy explains!

The straight part
We know that "Bluff/chap" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bluff/chap = cliff.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 CLIFF mean?
CLIFF noun- A vertical (or nearly vertical) rock face.
- A point where something abruptly fails or decreases in value etc.
- A symbol found on a musical staff that indicates the pitches represented by the lines and the spaces on the staff