Thick-skinned beast almost over English river (5)
I believe the answer is:
rhine
'river' is the definition.
(I know that Rhine is a river)
'thick-skinned beast almost over english' is the wordplay.
'thick-skinned beast' becomes 'rhino' (I've seen this before).
'almost' means to remove the last letter (almost the whole word is kept).
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'english' becomes 'E' (abbreviation).
'rhino' with its last letter taken away is 'rhin'.
'rhin'+'e'='RHINE'
(Other definitions for rhine that I've seen before include "River, rising in Switzerland and flowing into the North Sea" , "Flower" , "Bonn's river" , "River rising in Alps, with mouth at Rotterdam" , "River flowing from Switzerland, through Germany, to the North Sea" .)