Tree totally covers entrance to London thoroughfare (4,4)
I believe the answer is:
'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'.
'palmall' enclosing 'l' is 'PALL-MALL'.
(Other definitions for pall mall that I've seen before include "Monopoly board includes this", "Capital street", "thoroughfare", "Game once played", "London street with many private clubs".)