Establish there's nothing in cash (5)
I believe the answer is:
found
'establish' is the definition.
(found can mean to set up or establish)
'nothing in cash' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'in' indicates putting letters inside.
'cash' becomes 'fund' (as in a fund of cash).
'o' put within 'fund' is 'FOUND'.
'there's' acts as a link.
(Other definitions for found that I've seen before include "Discovered" , "Establish" , "Launch" , "To bring into being; cast" , "Start" .)