Second piece of bacon (4)
I believe the answer is:
back
This clue is a double definition.
'second' is the first definition.
(to second a motion is to back it)
'piece of bacon' is the second definition.
(as in back bacon)
(Other definitions for back that I've seen before include "See 17" , "Support a play - player" , "player in line of defence?" , "Stern" , "Support; in return" .)