Surround completely (7)
(Other definitions for enclose that I've seen before include "Totally surround" , "Encompass" , "Put inside" , "Cover" , "Fence in (common land)" .)
'surround completely' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for envelop that I've seen before include "Give all-round cover" , "Shroud, cover up" , "Enclose" , "Surround and embrace" , "Cover by wrapping" .)