The most common and most likely answer to this crossword clue is the 13 plus 3 letter word TYRANNOSAURUS REX
I've seen this crossword clue in Independent
T
Y
R
A
N
N
O
S
A
U
R
U
S
R
E
X
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.