The most common and most likely answer to this crossword clue is the 3 plus 2 plus 4 plus 6 letter word LAW OF MASS ACTION
I've seen this crossword clue in Financial Times
L
A
W
O
F
M
A
S
S
A
C
T
I
O
N
Buddy explains!
The straight part
We know that "Chemists’ fundamental" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case chemists’ fundamental = law of mass action.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.