Fast river paid off (5)
I believe the answer is:
rapid
'fast' is the definition.
(similar in meaning)
'river paid off' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'off' indicates an anagram.
'paid' anagrammed gives 'apid'.
'r'+'apid'='RAPID'
(Other definitions for rapid that I've seen before include "Fleet, swift" , "Happening quickly" , "Happening at great speed" , "Waterway where the current runs very fast" , "Fast, speedy" .)