The most common and most likely answer to this crossword clue is the 5 letter word PETTY
I've seen this crossword clue in The Guardian
P
E
T
T
Y
More clues leading to the result PETTY
- Small-minded, rather lacking in character, ultimately
- Miss Right, very small
- Attractive Romeo ditched as small-minded
- Insignificant conclusion to booklet in badly set type
- Junior‘s favourite toy’s lost its stuffing
- Favourite theory, being extremely selective? It’s not important
- Sulk, extremely testy and small-minded
- Run away from lovely Mickey Mouse
- Take right away from attractive rock star
- Rock star of no importance
Buddy explains!

The straight part
We know that "Junior" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case junior = petty.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 PETTY mean?
PETTY noun- (usually in the plural) A little schoolboy, either in grade or size.
- A class or school for young schoolboys.
- An outhouse: an outbuilding used as a lavatory.
- Little, small, secondary in rank or importance.
- Insignificant, trifling, or inconsiderable.
- Narrow-minded, small-minded.
- Begrudging in nature, especially over insignificant matters.