The most common and most likely answer to this crossword clue is the 7 plus 2 plus 10 letter word NOTHING IS IMPOSSIBLE
I've seen this crossword clue in The Guardian
N
O
T
H
I
N
G
I
S
I
M
P
O
S
S
I
B
L
E
Buddy explains!
The straight part
We know that "Optimistic assertion" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case optimistic assertion = nothing is impossible.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.