One may go up in a few ticks (4,4)
I believe the answer is:
time bomb
'one' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for time bomb that I've seen before include "Its detonation is delayed" , "Thing set to explode" , "Exploding ticker?" , "weapon" , "Delayed explosive" .)