The most common and most likely answer to this crossword clue is the 4 letter word YEAH
I've seen this crossword clue in The Times
Y
E
A
H
More clues leading to the result YEAH
- It's OK over in outskirts of Hackney
- Non-standard agreement by UEFA, tho’ consistently overlooked
- Ultimately, they conclude agenda with informal agreement
- Forage around, conserving energy for sure
- European visiting a hospital after hostel is very well
- A little stye a hindrance, certainly
- Certainly scrappy with Singapore, India and Bangladesh all at the back
- After 12 months, what about a positive response?
- Certainly, you will need a hospital
- Given period of time, husband at last agreed
Buddy explains!

The straight part
We know that "informal agreement" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case informal agreement = yeah.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 YEAH mean?
YEAH interjection- Expressing joy, celebration, glee, etc.
- A solar year, the time it takes the Earth to complete one revolution of the Sun (between 365.24 and 365.26 days depending on the point of reference).
- (by extension) The time it takes for any astronomical object (such as a planet, dwarf planet, small Solar System body, or comet) in direct orbit around a star (such as the Sun) to make one revolution around the star.
- A period between set dates that mark a year, from January 1 to December 31 by the Gregorian calendar, from Tishiri 1 to Elul 29 by the Jewish calendar, and from Muharram 1 to Dhu al-Hijjah 29 or 30 by the Islamic calendar.
- A scheduled part of a calendar year spent in a specific activity.
- A Julian year, exactly 365.25 days, represented by "a".
- A level or grade in school or college.
- The proportion of a creature's lifespan equivalent to one year of an average human lifespan (see also dog year).