The most common and most likely answer to this crossword clue is the 6 letter word ROSIER
I've seen this crossword clue in The Times
R
O
S
I
E
R
Buddy explains!

The straight part
We know that "More optimistic" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case more optimistic = rosier.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 ROSIER mean?
ROSIER adjective- Rose-coloured.
- Resembling rose, as in scent of perfume.
- Optimistic.
- Rosebush