Mammal in favour of rising in dignity (8)
I believe the answer is:
porpoise
'mammal' is the definition.
(I've seen this before)
'in favour of rising in dignity' is the wordplay.
'in favour of' becomes 'pro' (I've seen this before).
'rising' says the letters should be written in reverse (going up in a down clue).
'in' is an insertion indicator.
'dignity' becomes 'poise'.
'pro' back-to-front is 'orp'.
'orp' going into 'poise' is 'PORPOISE'.
(Other definitions for porpoise that I've seen before include "Member of dolphin family" , "Sea beast" , "Sea creature (reported close behind by the 10 to the 9!)" , "Small aquatic mammal with blunt snout" , "Type of dolphin" .)