The most common and most likely answer to this crossword clue is the 8 plus 6 letter word UNLEADED PETROL
I've seen this crossword clue in Financial Times
U
N
L
E
A
D
E
D
P
E
T
R
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L
More clues leading to the result UNLEADED PETROL
Buddy explains!
The straight part
We know that "It’s free from Pb" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s free from pb = unleaded petrol.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.