Means of support (4)
(Other definitions for legs that I've seen before include "Sections of a journey or course" , "Lower limbs" , "Stages of a journey" , "associated with 11 in game" , "Members" .)
'means of support' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for bread and butter that I've seen before include "pudding" , "basically reliable" , "Means of support" , "Food" , "rather monotonous diet" .)