Animal mostly seen around East German river (5)
I believe the answer is:
rhein
'german river' is the definition.
(I've seen this in another clue)
'animal mostly seen around east' is the wordplay.
'animal' becomes 'rhino' (short for rhinoceros).
'mostly' means to remove the last letter (most but not all of the word).
'seen around' is an insertion indicator.
'east' becomes 'e' (abbreviation).
'rhino' with its final letter removed is 'rhin'.
'rhin' placed around 'e' is 'RHEIN'.
(Other definitions for rhein that I've seen before include "You might find her in the river, strangely" , "(German name for) German river" , "Locally busy European river" , "In Germany, find her in the river, strangely" .)