Sum of £1000 in cash (6)
I believe the answer is:
monkey
'sum' is the definition.
(monkey is slang for 500 pounds)
'ps1000 in cash' is the wordplay.
'psk000' becomes 'k' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'cash' becomes 'money' (cash is a type of money).
'k' put into 'money' is 'MONKEY'.
'of' is the link.
(Other definitions for monkey that I've seen before include "Any of various long-tailed primates" , "See 10" , "Mischievous child; primate" , "Mischievous type" , "Five hundred pounds (colloq.); imp" .)