The most common and most likely answer to this crossword clue is the 7 plus 3 plus 5 letter word COSTING THE EARTH
C
O
S
T
I
N
G
T
H
E
E
A
R
T
H
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.