Start of pussy cat's spring (6)
I believe the answer is:
pounce
'spring' is the definition.
(pounce is a kind of spring)
'start of pussy cat's' is the wordplay.
'start of pussy' becomes 'p' (I can't explain this - if you can you should believe this answer much more).
'cat' becomes 'ounce' (ounce is a kind of cat).
'p'+'ounce'='POUNCE'
Can you help me to learn more?
(Other definitions for pounce that I've seen before include "Strike" , "Spring down on and grasp" , "Swoop down suddenly" , "Spring or swoop down on to attack" , "Jump swiftly on, as cat might" .)