The most common and most likely answer to this crossword clue is the 4 plus 4 letter word HEAT PUMP
I've seen this crossword clue in The Times
H
E
A
T
P
U
M
P
More clues leading to the result HEAT PUMP
Buddy explains!
The straight part
We know that "warmer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case warmer = heat pump.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.