Hissing back when the cat's caught it (6)
I believe the answer is:
static
'hissing' is the definition.
(I've seen this before)
'back when the cat's caught it' is the wordplay.
'back' is a reversal indicator.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'the cat's' becomes 'tic' (I can't explain this - if you can you should give a lot more credence to this answer).
'caught' indicates putting letters inside.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'as' back-to-front is 'sa'.
'sa'+'tic'='satic'
'satic' enclosing 't' is 'STATIC'.
Can you help me to learn more?
(Other definitions for static that I've seen before include "showing no signs of going anywhere" , "Not active or moving" , "Its cat (anag)" , "Unmoving, unchanging" , "Inactive or inert" .)