The most common and most likely answer to this crossword clue is the 8 letter word MISTRESS
I've seen this crossword clue in The Daily Telegraph
M
I
S
T
R
E
S
S
More clues leading to the result MISTRESS
- Teacher, one with tension after end of term
- Girl cut hair: she wears the trousers?
- Note strain for lover
- Woman on side of motorway subject to strain
- Heading off trouble with money, making a bit on the side
- Worry after change of head teacher
- Lover puts weight on next
- Male resists changing fancy woman
- Maiden, one facing emotional pressure as ‘the other woman’?
- Girl embracing very French lover
Buddy explains!

The straight part
We know that "‘the other woman’" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ‘the other woman’ = mistress.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 MISTRESS mean?
MISTRESS noun- A woman, specifically one with great control, authority or ownership
- A female teacher
- The other woman in an extramarital relationship, generally including sexual relations
- A dominatrix
- A woman well skilled in anything, or having the mastery over it
- A woman regarded with love and devotion; a sweetheart
- A married woman; a wife
- The jack in the game of bowls
- A female companion to a master (a man with control, authority or ownership)
- Of a woman: to master; to learn or develop to a high degree of proficiency.
- To act or take the role of a mistress.