In the end, Los Angeles street starts to look yellow (6)
I believe the answer is:
'in the end' is the definition.
(I've seen this before)
'los angeles street starts to look yellow' is the wordplay.
'los angeles' becomes 'la'.
'street' becomes 'st' (abbreviation).
'starts to' says to take the initial letters.
The initial letters of 'look yellow' is 'ly'.
(Other definitions for lastly that I've seen before include "In conclusion" , "In the end" , "Finally, to conclude with" .)