Animal one’s kept in German city mostly (5)
I believe the answer is:
bison
'animal' is the definition.
(I've seen this before)
'one's kept in german city mostly' is the wordplay.
'one's' becomes 'is' (Roman numeral).
'kept in' is an insertion indicator.
'german city' becomes 'bonn' (city in Germany).
'mostly' means to remove the last letter (most but not all of the word).
'bonn' with its final letter removed is 'bon'.
'is' going inside 'bon' is 'BISON'.
(Other definitions for bison that I've seen before include "US wild ox" , "Wild ox of the prairies" , "Buffalo Bill's prey" , "Big beast" , "It's a buffalo in America" .)