Cat that will jump on the piano (5)
I believe the answer is:
ounce
'cat that will jump' is the definition.
'ounce' can be an answer for 'cat' (ounce is a kind of cat). I am not certain of the rest of the definition.
'on the piano' is the wordplay.
'on' becomes 'o' (I've seen this in other clues).
'the piano' becomes 'unce' (I can't justify this - if you can you should give a lot more credence to this answer).
'o'+'unce'='OUNCE'
Can you help me to learn more?
(Other definitions for ounce that I've seen before include "On cue (anag.)" , "scrap" , "Unit equal to about 28 grams" , "A light imperial measure of weight" , "Small imperial weight" .)