Mounting, note pale outside of river mammal (7)
I believe the answer is:
narwhal
'mammal' is the definition.
(narwhal is a kind of mammal)
'mounting note pale outside of river' is the wordplay.
'mounting' says the letters should be written in reverse (going up in a down clue).
'note' becomes 'lah' (I've seen this before).
'pale' becomes 'wan' (wan can mean pale or grey).
'outside of' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation for river).
'lah'+'wan'='lahwan'
'lahwan' enclosing 'r' is 'lahwran'.
'lahwran' backwards is 'NARWHAL'.
(Other definitions for narwhal that I've seen before include "Ocean mammal" , "Marine mammal, the male having a spiral ivory tusk" , "Animal with large spiral tusk" , "swimmer" , "Arctic toothed whale" .)