The most common and most likely answer to this crossword clue is the 4 plus 2 letter word CHIN UP
I've seen this crossword clue in The Daily Telegraph
C
H
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More clues leading to the result CHIN UP
Buddy explains!
The straight part
We know that "Keep optimistic" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case keep optimistic = chin up.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 CHIN UP mean?
CHIN UP noun- An exercise done for strengthening the arms and back, in which one lifts one's own weight while hanging from a bar. It can be defined more restrictively on criteria like the arms starting at lock-out, or the chin requiring contact with the bottom of the bar, or touching the neck to the bar.