The most common and most likely answer to this crossword clue is the 3 plus 2 plus 1 plus 5 plus 4 letter word PUT ON A BRAVE FACE
I've seen this crossword clue in The Guardian
P
U
T
O
N
A
B
R
A
V
E
F
A
C
E
Buddy explains!
The straight part
We know that "Keep your chin up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case keep your chin up = put on a brave face.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.