Kitty's up late — have really quick check? (4,4)
I believe the answer is:
stop dead
'really quick check?' is the definition.
'stop dead' can be an answer for 'check?' (thesaurus). I am not sure about the 'really quick' bit.
'kitty's up late' is the wordplay.
'kitty's' becomes 'pot's' (both can mean a shared fund).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'late' becomes 'dead' (similar in meaning).
'pots' reversed gives 'stop'.
'stop'+'dead'='STOP DEAD'
'have' is the link.
Can you help me to learn more?
(Other definitions for stop dead that I've seen before include "Pull up abruptly" , "Become immobilised" .)