Means of control (6)
'means of control' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for reins that I've seen before include "Power to direct and control" , "Checks" , "Means of control for a horse" , "Guiding straps" , "Control for horses" .)
(Other definitions for button that I've seen before include "fast driver?" , "One may invite the press to function" , "Knob for fastening" , "Round fastener" , "Push it or do it up!" .)