The most common and most likely answer to this crossword clue is the 8 letter word TWOPENCE
I've seen this crossword clue in Independent
T
W
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P
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More clues leading to the result TWOPENCE
Buddy explains!

The straight part
We know that "small sum" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case small sum = twopence.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 TWOPENCE mean?
TWOPENCE noun- A British or Irish coin worth two (old or new) pence.
- A cost or value of two pence.
- (by extension) A small amount or value.