The most common and most likely answer to this crossword clue is the 7 letter word LIGHTEN
I've seen this crossword clue in The Daily Telegraph
L
I
G
H
T
E
N
More clues leading to the result LIGHTEN
- Reduce awkward length, squeezing one in
- Reduce bleach
- Make less burdensome and so make fairer
- Reduce one in length, possibly
- Reduce impact of disease, sending away first nurse
- Temper evident when sea captain loses head in front of a number
- Dawn is close, with a new beginning
- If missing nightlife, lost temper
Buddy explains!

The straight part
We know that "temper" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case temper = lighten.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 LIGHTEN mean?
LIGHTEN verb- To make brighter or clearer; to illuminate.
- To become brighter or clearer; to brighten.
- To burst forth or dart, as lightning; to shine with, or like, lightning; to flash.
- To emit or disclose in, or as if in, lightning; to flash out, like lightning.
- To illuminate with knowledge; to enlighten.
- To alleviate; to reduce the burden of.
- To make light or lighter in weight.
- To make less serious or more cheerful.
- To become light or lighter in weight.
- To become less serious or more cheerful.
- To descend; to light.