Go fast: get to river after second (5)
I believe the answer is:
motor
'go fast' is the definition.
'motor' can be an answer for 'go' (motoring is a kind of going). I'm unsure of the 'fast' bit.
'to river after second' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'after' means one lot of letters go next to another.
'second' becomes 'mo' (both are short periods of time).
'to'+'r'='tor'
'tor' put after 'mo' is 'MOTOR'.
'get' is the link.
Can you help me to learn more?
(Other definitions for motor that I've seen before include "Engine - relating to movement" , "Car; powered machine" , "Driver" , "Car (colloq,)" , "Drive" .)