Energy of ducks on speed (5)
I believe the answer is:
oomph
'energy' is the definition.
(oomph can mean energy or excitement)
'ducks on speed' is the wordplay.
'ducks' becomes 'OO' (two O's - duck means zero in cricket).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'speed' becomes 'mph' (abbreviation for miles per hour).
'oo'+'mph'='OOMPH'
'of' is the link.
(Other definitions for oomph that I've seen before include "Go" , "Exciting energy" , "Informal energy or enthusiasm, usually in a girl" , "sex-appeal" , "vital quality" .)