To get two ducks at speed requires vigour (5)
I believe the answer is:
oomph
'vigour' is the definition.
(I have seen 'Spirited vigour' mean 'oomph' so perhaps 'vigour' could also mean 'oomph')
'to get two ducks at speed' is the wordplay.
'two ducks' can mean two replacements for 'duck'.
'to get' means one lot of letters go next to another.
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'at' says to put letters next to each other.
'speed' becomes 'mph' (abbreviation for miles per hour).
'o'+'o' is 'oo'.
'oo'+'mph'='OOMPH'
'requires' acts as a link.
(Other definitions for oomph that I've seen before include "Spirited vigour" , "Shows excitement aplenty" , "Informal energy or enthusiasm, usually in a girl" , "Ginger" , "vital quality" .)