Extensive (4)

'extensive' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for vast that I've seen before include "Gigantic" , "Huge, enormous" , "Unusually great" , "'Huge, immense (4)'" , "Of very great extent" .)
(Other definitions for wall-to-wall that I've seen before include "(Of carpet) fitted" , "Widespread" .)