Useless demanding £1000 plus a deposit (5)
I believe the answer is:
kaput
'useless' is the definition.
(something kaput is broken or unable to function)
'ps1000 plus a deposit' is the wordplay.
'psk000' becomes 'ka' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'plus' says to put letters next to each other.
'a deposit' becomes 'put' (I've seen this in other clues).
'ka'+'put'='KAPUT'
'demanding' is the link.
(Other definitions for kaput that I've seen before include "Broken down irretrievably" , "Broken or finished - in Germany" , "Defunct (informal)" , "Broken and useless" , "Broken (sl.)" .)