The most common and most likely answer to this crossword clue is the 11 letter word ROSE-TINTED
I've seen this crossword clue in Independent
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More clues leading to the result ROSE-TINTED
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The straight part
We know that "Overly optimistic" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case overly optimistic = rose-tinted.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.
Crossword Answer definitions
ROSE-TINTED verb- To look through rose-tinted glasses at; to view or describe as better than it actually is or was.
- To tint with the color rose; to suffuse with a rosy hue.