The most common and most likely answer to this crossword clue is the 3 letter word FUN
I've seen this crossword clue in The Guardian
F
U
N
More clues leading to the result FUN
Buddy explains!

The straight part
We know that "Good time" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case good time = fun.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 FUN mean?
FUN noun- Amusement, enjoyment or pleasure
- Playful, often noisy, activity.
- To tease, kid, poke fun at, make fun of.
- Enjoyable, amusing
- Whimsical, flamboyant