Second hired car sent over at the weekend (4)
I believe the answer is:
back
'second' is the definition.
(to second a motion is to back it)
'hired car sent over at the weekend' is the wordplay.
'hired car' becomes 'cab' (I've seen this before).
'sent over' shows that the letters should be reversed in order.
'at' says to put letters next to each other.
'the weekend' becomes 'k' (I've seen this in other clues).
'cab' reversed gives 'bac'.
'bac'+'k'='BACK'
(Other definitions for back that I've seen before include "Support; in return" , "player in line of defence?" , "Stern" , "Help!" , "part of the body" .)