The most common and most likely answer to this crossword clue is the 15 letter word BACTERIOLOGICAL
I've seen this crossword clue in The Daily Telegraph
B
A
C
T
E
R
I
O
L
O
G
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C
A
L
Buddy explains!
The straight part
We know that "concerned with microscopic things" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case concerned with microscopic things = bacteriological.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 BACTERIOLOGICAL mean?
BACTERIOLOGICAL adjective- Of or pertaining to bacteriology.