The most common and most likely answer to this crossword clue is the 6 letter word TEMPER
I've seen this crossword clue in Financial Times
T
E
M
P
E
R
More clues leading to the result TEMPER
- Metaphor turned on a lime, say
- What is shown by irate MP erupting?
- Harden mood
- Heading north, salesperson encountered anger
- Travelling salesman encountered getting up in a bad mood
- Moderate politician divides support on right . . . .
- Moderate salesman paid up
- Soften, harden
- Harden or soften?
- Style of painting ultimately lacking self-control
Buddy explains!

The straight part
We know that "Harden/soften" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case harden/soften = temper.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 TEMPER mean?
TEMPER noun- A tendency to be in a certain type of mood; a habitual way of thinking, behaving or reacting.
- State of mind; mood.
- A tendency to become angry.
- Anger; a fit of anger.
- Calmness of mind; moderation; equanimity; composure.
- Constitution of body; the mixture or relative proportion of the four humours: blood, choler, phlegm, and melancholy.
- Middle state or course; mean; medium.
- The state of any compound substance which results from the mixture of various ingredients; due mixture of different qualities.
- The heat treatment to which a metal or other material has been subjected; a material that has undergone a particular heat treatment.
- The state of a metal or other substance, especially as to its hardness, produced by some process of heating or cooling.
- (sugar manufacture) Milk of lime, or other substance, employed in the process formerly used to clarify sugar.
- To moderate or control.
- To strengthen or toughen a material, especially metal, by heat treatment; anneal.
- To sauté spices in ghee or oil to release essential oils for flavouring a dish in South Asian cuisine.
- To mix clay, plaster or mortar with water to obtain the proper consistency.
- To adjust, as the mathematical scale to the actual scale, or to that in actual use.
- (Latinism) To govern; to manage.
- To combine in due proportions; to constitute; to compose.
- To mingle in due proportion; to prepare by combining; to modify, as by adding some new element; to qualify, as by an ingredient; hence, to soften; to mollify; to assuage.
- To fit together; to adjust; to accommodate.