The most common and most likely answer to this crossword clue is the 3 plus 3 plus 3 letter word TIT FOR TAT
I've seen this crossword clue in The Guardian
T
I
T
F
O
R
T
A
T
More clues leading to the result TIT FOR TAT
- Note rubbish bags keep Cockney’s hat
- Retaliation: what makes ‘hit a toff’ click!?
- Boob, allowing soldiers to penetrate stronghold in retaliation
- Revenge makes tart fit to burst
- Attempt to pay back fat trio swimming in Tees, you might say
- Make tart fit to give in return
- Trait oft misconstrued, time for revenge
- Tense appeal with regards to rag causing retaliation
- … pork-pie maybe gives satisfaction
- Fit to bust, tart somehow gets revenge
Buddy explains!
The straight part
We know that "principle of The Avengers" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case principle of the avengers = tit for tat.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.