Thick-skinned beast dropping back by European river (5)
I believe the answer is:
rhine
'river' is the definition.
(I know that Rhine is a river)
'thick-skinned beast dropping back by european' is the wordplay.
'thick-skinned beast' becomes 'rhino' (I've seen this before).
'dropping back' means to remove the last letter (the letter at the back is removed).
'by' says to put letters next to each other.
'european' becomes 'E' (abbreviation e.g. EU).
'rhino' with its last letter removed is 'rhin'.
'rhin'+'e'='RHINE'
(Other definitions for rhine that I've seen before include "water" , "fed by 8 [RHUR]" , "River of Cologne" , "Great European river with Lorelei Rock" , "Great river that flows through Germany" .)