Cat under pressure will jump (5)
I believe the answer is:
ounce
'cat' is the definition.
(ounce is a kind of cat)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for ounce that I've seen before include "Unit of weight; cat" , "A little bit" , "28.3495 grammes" , "On cue (anag.)" , "weight? Surely not" .)