On supplier's van, prompt delivery (6)
I believe the answer is:
rescue
'delivery' is the definition.
(I know that delivery can be written as rescue)
'on supplier's van prompt' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'van' says to take the initial letters (van is the front part of something).
'prompt' becomes 'cue' (cue can mean a prompt or signal).
The initial letter of 'suppliers' is 's'.
're'+'s'+'cue'='RESCUE'
(Other definitions for rescue that I've seen before include "Save from harm" , "Secure (anag) - release" , "Pull from danger" , "Save from danger or distress" , "liberation" .)