The most common and most likely answer to this crossword clue is the 8 letter word CLIMBING
I've seen this crossword clue in The Guardian
C
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M
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Buddy explains!
The straight part
We know that "on the up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case on the up = climbing.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 CLIMBING mean?
CLIMBING verb- To ascend; rise; to go up.
- To mount; to move upwards on.
- To scale; to get to the top of something.
- To move (especially up and down something) by gripping with the hands and using the feet.
- To practise the sport of climbing
- To jump high
- To move to a higher position on the social ladder.
- Of plants, to grow upwards by clinging to something.
- The sport of climbing, ascending a wall or a rock or another object using available holds, generally with the safety of a rope and belayer.
- Climb; ascent.
- (of a plant) That climbs; that grows upwards by gripping onto a surface.