The most common and most likely answer to this crossword clue is the 1 plus 6 plus 3 letter word A SINGLE MAN
I've seen this crossword clue in The Guardian
A
S
I
N
G
L
E
M
A
N
Buddy explains!
The straight part
We know that "chap lacking a partner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case chap lacking a partner = a single man.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.