Heavy rain initially delayed every London Underground going east (6)
I believe the answer is:
deluge
'heavy rain' is the definition.
(I've seen this before)
'initially delayed every london underground going east' is the wordplay.
'initially' says to take the initial letters.
'east' becomes 'e' (synonyms).
The first letters of 'delayed every london underground going' is 'delug'.
'delug'+'e'='DELUGE'
(Other definitions for deluge that I've seen before include "Overwhelm with water" , "Cloudburst, downpour" , "Severe flood" , "Huge downpour" , "Torrential rain" .)