The most common and most likely answer to this crossword clue is the 4 plus 8 letter word SWIM UPSTREAM
I've seen this crossword clue in Financial Times
S
W
I
M
U
P
S
T
R
E
A
M
Buddy explains!
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.