Monster one found in ascendant German city (6)
I believe the answer is:
nessie
'monster' is the definition.
(I know that Nessie is a legendary creature)
'one found in ascendant german city' is the wordplay.
'one' becomes 'i' (Roman numeral).
'found in' means one lot of letters goes inside another.
'ascendant' is a reversal indicator (in a down clue, going upwards).
'german city' becomes 'essen' (city in Germany).
'essen' backwards is 'nesse'.
'i' placed within 'nesse' is 'NESSIE'.
(Other definitions for nessie that I've seen before include "Supposed loch monster" , "Scots loch monster (colloq.)" , "Nickname for a Scottish monster" .)