Old East German follows river to find perch (5)
I believe the answer is:
roost
'perch' is the definition.
('roost' can be a synonym of 'perch')
'old east german follows river' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'east german' becomes 'ost' ('east' in German).
'follows' means one lot of letters go next to another.
'river' becomes 'r'.
'o'+'ost'='oost'
'oost' put after 'r' is 'ROOST'.
'to find' is the link.
(Other definitions for roost that I've seen before include "Temporary resting-place" , "Resting perch for birds" , "Perch on a branch" , "Shetland tidal water" , "Perch where bird sleeps" .)