The most common and most likely answer to this crossword clue is the 8 letter word MANTISSA
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The straight part
We know that "Bit of a log" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bit of a log = mantissa.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 MANTISSA mean?
MANTISSA noun- A minor addition to a text.
- The part of a common logarithm after the decimal point, the fractional part of a logarithm.
- The significand; that part of a floating-point number or number in scientific notation that contains its significant digits.