Very popular time to leave part of London (4)
I believe the answer is:
soho
'part of london' is the definition.
(district of Westminster)
'very popular time to leave' is the wordplay.
'very' becomes 'so' (both can mean 'extremely').
'popular' becomes 'hot' (hot can mean popular or trendy).
'time' becomes 't' (abbreviation).
'to leave' suggests deleting specific letters.
'hot' with 't' taken away is 'ho'.
'so'+'ho'='SOHO'
(Other definitions for soho that I've seen before include "Colourful neighbourhood in London's West End" , "Colourful district in central London" , "Once notorious part of central London" , "London area renowned for nightlife" , "Smoke's part" .)