Tree totally covers entrance to London thoroughfare (4,4)
I believe the answer is:
pall mall
'thoroughfare' is the definition.
(I've seen this before)
'tree totally covers entrance to london' is the wordplay.
'tree' becomes 'palm' (palm tree is a kind of tree).
'totally' becomes 'all'.
'covers' is an insertion indicator.
'entrance to' indicates taking the first letters.
The initial letter of 'london' is 'l'.
'palm'+'all'='palmall'
'palmall' enclosing 'l' is 'PALL-MALL'.
(Other definitions for pall mall that I've seen before include "Monopoly board includes this" , "London street (worth PS140 in 14)" , "Sixteenth and seventeenth century ball game" , "Game once played" , "Old ball game" .)