The most common and most likely answer to this crossword clue is the 12 letter word BLOCKBUSTERS
I've seen this crossword clue in The Daily Telegraph
B
L
O
C
K
B
U
S
T
E
R
S
Buddy explains!
The straight part
We know that "They are very successful" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they are very successful = blockbusters.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 BLOCKBUSTERS mean?
BLOCKBUSTERS noun- A high-explosive bomb used for the purposes of demolishing extensive areas, such as a city block.
- Something, such as a film or book, that sustains exceptional and widespread popularity and achieves enormous sales, as opposed to a box office bomb.
- Anything very large or powerful; a whopper.
- A large firework of the firecracker type; an M-80.
- One who engages in blockbusting (technique encouraging people to sell property).