The most common and most likely answer to this crossword clue is the 1 plus 3 plus 2 plus 3 letter word A BOB OR TWO
I've seen this crossword clue in Financial Times
A
B
O
B
O
R
T
W
O
More clues leading to the result A BOB OR TWO
Buddy explains!
The straight part
We know that "quite a hefty sum?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case quite a hefty sum? = a bob or two.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.