The most common and most likely answer to this crossword clue is the 3 letter word HIM
I've seen this crossword clue in The Times
H
I
M
More clues leading to the result HIM
Buddy explains!
The straight part
We know that "That chap" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that chap = him.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 HIM mean?
HIM pronoun- Honorific alternative letter-case form of him, sometimes used when referring to God or another important figure who is understood from context.
- A male person.
- A masculine pronoun; he as a grammatical object.
- Used reflexively: (to) himself.
- With nominative effect: he, especially as a predicate after be, or following a preposition.