Cast will turn up in Cats , so there! (4)
I believe the answer is:
toss
'cast' is the definition.
(I've seen this in another clue)
'turn up in cats so there' is the wordplay.
'turn up' says the letters should be written backwards.
'in' indicates a hidden word.
'ssot' is hidden within 'cats so there'.
'ssot' in reverse letter order is 'TOSS'.
'will' is the link.
(Other definitions for toss that I've seen before include "have a restless night, perhaps" , "Chuck" , "when ship does this?" , "What to do with a pancake and a caber" , "Fling" .)