Prompt agent to cross river from the south (6)
I believe the answer is:
speedy
'prompt' is the definition.
(I've seen this in another clue)
'agent to cross river from the south' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'to cross' is an insertion indicator (some letters cross or go round others).
'river' becomes 'dee' (River Dee).
'from the south' says the letters should be written backwards (in a down clue, letters go up).
'dee' in reverse letter order is 'eed'.
'spy' placed around 'eed' is 'SPEEDY'.
(Other definitions for speedy that I've seen before include "Expeditious" , "in a hurry" , "Express" , "Quick" , "Fast-moving" .)