Agent going round ebbing river fast (6)
I believe the answer is:
speedy
'fast' is the definition.
(similar in meaning)
'agent going round ebbing river' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'going round' means one lot of letters goes inside another (some letters go around others).
'ebbing' is a reversal indicator (to ebb can mean to go backwards).
'river' becomes 'Dee' (River Dee).
'dee' written backwards gives 'eed'.
'spy' going around 'eed' is 'SPEEDY'.
(Other definitions for speedy that I've seen before include "Done quickly" , "Express" , "Rapid, fast" , "in a hurry" , "in a fast car?" .)