On way back deliver £1000 weapon (4)
I believe the answer is:
dirk
'weapon' is the definition.
(dirk is a kind of weapon)
'on way back deliver ps1000' is the wordplay.
'on way back' says the letters should be written in reverse.
'deliver' becomes 'rid' (to rid from is to free or deliver from).
'psk000' becomes 'k' (I am not sure about this - if you are sure you should believe this answer much more).
'rid' back-to-front is 'dir'.
'dir'+'k'='DIRK'
Can you help me to learn more?
(Other definitions for dirk that I've seen before include "Small dagger" , "Scots dagger" , "weapon" , "Duncan's killer?" , "Dagger (belonging to Bogarde?)" .)