Lobby leaders in attempt to rebuild infrastructure using machines (6)
I believe the answer is:
atrium
'lobby' is the definition.
(I've seen this before)
'leaders in attempt to rebuild infrastructure using machines' is the wordplay.
'leaders in' indicates taking the first letters.
The first letters of 'attempt to rebuild infrastructure using machines' is 'ATRIUM'.
(Other definitions for atrium that I've seen before include "Central hall of shopping mall etc" , "Central court of old Roman house, or heart cavity" , "An upper cavity in the heart" , "Um, I rat on the chamber of the heart" , "Central chamber in an ancient Roman house" .)