The most common and most likely answer to this crossword clue is the 7 letter word TEACHER
I've seen this crossword clue in The Times
T
E
A
C
H
E
R
More clues leading to the result TEACHER
- Yank grabbing Conservative fellow’s mistress?
- Suffer during most of term? I might
- School worker sees a head in term, mostly
- One may be content to educate a cherub
- Something brewed by singer that’s instructive type
- A paid informer
- Drink with singer and guide
- Cheater embarrassed tutor
- Lecturer exhibiting pain during short semester
- He may be a master of form
Buddy explains!

The straight part
We know that "mistress" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mistress = teacher.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 TEACHER mean?
TEACHER noun- A person who teaches, especially one employed in a school.
- The index finger; the forefinger.
- An indication; a lesson.
- The second highest office in the Aaronic priesthood, held by priesthood holders of at least the age of 14.