The most common and most likely answer to this crossword clue is the 16 letter word STICKING-PLASTER
I've seen this crossword clue in The Times
S
T
I
C
K
I
N
G
-
P
L
A
S
T
E
R
Buddy explains!
The straight part
We know that "Item in first-aid kit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case item in first-aid kit = sticking-plaster.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.