Thus laugh in London area (4)
I believe the answer is:
soho
'london area' is the definition.
(I've seen this before)
'thus laugh' is the wordplay.
'thus' becomes 'so' (synonyms).
'laugh' becomes 'ho' (representation of a laughing sound).
'so'+'ho'='SOHO'
'in' is the link.
(Other definitions for soho that I've seen before include "London's nightlife centre" , "foreign quarter" , "London area; hunt call" , "London area renowned for nightlife" , "(Formerly) disreputable area of London" .)