Jump on soft weight (6)
I believe the answer is:
pounce
'jump on' is the definition.
(thesaurus)
'soft weight' is the wordplay.
'soft' becomes 'p' (musical abbreviation for 'piano').
'weight' becomes 'ounce' (ounce is a kind of weight).
'p'+'ounce'='POUNCE'
(Other definitions for pounce that I've seen before include "Jump down on suddenly" , "Spring (on prey like a cat)" , "(Of cat) sudden leap to seize prey" , "Swoop down suddenly" , "Jump or spring down as on prey" .)