Second defender (4)
I believe the answer is:
back
I believe this clue is a double definition.
'second' is the first definition.
(to second a motion is to back it)
'defender' is the second definition.
(I've seen this before)
(Other definitions for back that I've seen before include "Stern" , "Support; in return" , "Second" , "Rear; spine" , "Support (from behind?)" .)