Key people mostly returning to support German city (9)
I believe the answer is:
essential
'key' is the definition.
(I've seen this before)
'people mostly returning to support german city' is the wordplay.
'people mostly' becomes 'lait' (I am not sure about this - if you are sure you should believe this answer much more).
'returning' is a reversal indicator.
'to support' is a charade indicator (letters next to each other) (in a down clue, some letters hold up others).
'german city' becomes 'essen' (city in Germany).
'lait' backwards is 'tial'.
'tial' put after 'essen' is 'ESSENTIAL'.
Can you help me to learn more?
(Other definitions for essential that I've seen before include "Indispensible" , "organic" , "as oil might be" , "absolutely neccessary?" , "Vital" .)