The most common and most likely answer to this crossword clue is the 5 letter word TEPID
I've seen this crossword clue in The Times
T
E
P
I
D
More clues leading to the result TEPID
- Fellow eating most of baked dish, barely warm
- Returned pet I had with little enthusiasm
- Papa stuck into council — flipping cool
- Cool Dublin politician holds record, first in Ireland
- Unenthusiastic in assembly over bringing in piano
- Lukewarm fare served up quietly inside
- Edward keeps relationship lukewarm
- Unenthusiastic in assembly about bringing in piano
- Lukewarm food served up, including pathetic starter
- What’s a bit warm and soft in selection of food served up
Buddy explains!

The straight part
We know that "Cool" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cool = tepid.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 TEPID mean?
TEPID adjective- Lukewarm; neither warm nor cool.
- Uninterested; exhibiting little passion or eagerness.