The most common and most likely answer to this crossword clue is the 9 plus 6 letter word IMMERSION HEATER
I've seen this crossword clue in The Daily Telegraph
I
M
M
E
R
S
I
O
N
H
E
A
T
E
R
More clues leading to the result IMMERSION HEATER
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.