Vital German city -- it rises above a lake (9)
I believe the answer is:
essential
'vital' is the definition.
('essential' can be similar in meaning to 'vital')
'german city it rises above a lake' is the wordplay.
'german city' becomes 'essen' (I've seen this before).
'rises' shows that the letters should be reversed in order.
'above' says to put letters next to each other.
'lake' becomes 'l'.
'it' written backwards gives 'ti'.
'essen'+'ti'+'a'+'l'='ESSENTIAL'
(Other definitions for essential that I've seen before include "must-have item" , "Type of oil" , "So is certain oil-production" , "Key" , "organic" .)