Watch flood engulfing City before end of August (8)
I believe the answer is:
spectate
'watch' is the definition.
(spectating is a kind of watching)
'flood engulfing city before end of august' is the wordplay.
'flood' becomes 'spate' ('spate' can be a synonym of 'flood'**).
'engulfing' indicates putting letters inside.
'city' becomes 'ec' (the City of London postcode).
'before' says to put letters next to each other.
'end of' suggests the final letters.
The final letter of 'august' is 't'.
'ec'+'t'='ect'
'spate' going around 'ect' is 'SPECTATE'.
(Other definitions for spectate that I've seen before include "Watch (eg, a game)" , "Be an onlooker" , "Watch (at sports event)" , "Observe" , "Be a viewer, at a sports event perhaps" .)