The most common and most likely answer to this crossword clue is the 14 letter word WELL-COLLECTED
W
E
L
L
-
C
O
L
L
E
C
T
E
D
Buddy explains!
The straight part
We know that "such specimens are sure to be in a good museum" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case such specimens are sure to be in a good museum = well-collected.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.