The most common and most likely answer to this crossword clue is the 4 letter word PEEN
I've seen this crossword clue in Independent
P
E
E
N
More clues leading to the result PEEN
Buddy explains!

The straight part
We know that "bit of a tool" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bit of a tool = peen.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 PEEN mean?
PEEN noun- The (often spherical) end of the head of a hammer opposite the main hammering end.
- To shape metal by striking it, especially with a peen.
- Penis.