Important German city -- it is round a lake (9)
I believe the answer is:
essential
'important' is the definition.
('essential' can be similar in meaning to 'important')
'german city it is round a lake' is the wordplay.
'german city' becomes 'essen' (I've seen this before).
'is round' says the letters should be written backwards.
'lake' becomes 'l'.
'it' written backwards gives 'ti'.
'essen'+'ti'+'a'+'l'='ESSENTIAL'
(Other definitions for essential that I've seen before include "Indispensable, necessary" , "something necessary" , "must-have item" , "Crucial" , "Basic or necessary" .)