So is flood regularly seen in capital city? (4)
I believe the answer is:
oslo
'capital city?' is the definition.
(I've seen this before)
'so is flood regularly seen' is the wordplay.
'regularly seen' indicates alternate letters.
'so'+'is'+'flood'='soisflood'
The alternating letters of 'soisflood' are 'OSLO'.
'in' acts as a link.
(Other definitions for oslo that I've seen before include "Norwegian chief port" , "Nordic city" , "City in Norway" , "European capital, known until 1924 as Christiania" , "foreign port" .)