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' says the letters should be written in reverse.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'the cat's' becomes 'tic' (I can't justify 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' written backwards gives '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" .)