The most common and most likely answer to this crossword clue is the 12 letter word FAST-BREEDER
I've seen this crossword clue in The Times
F
A
S
T
-
B
R
E
E
D
E
R
Buddy explains!
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.