Strangely upholding skinless mammal, in the main (7)
I believe the answer is:
dolphin
'mammal in the main' is the definition.
(sea-dwelling mammal - the main can mean the sea)
'strangely upholding skinless' is the wordplay.
'strangely' is an anagram indicator.
'skinless' means to look at the middle letters.
The middle letters of 'upholding' are 'pholdin'.
'pholdin' is an anagram of 'DOLPHIN'.
(Other definitions for dolphin that I've seen before include "Sea mammal like large porpoise, friendly towards people" , "Sea creature" , "Marine mammal with an elongated beak-like head" , "swimmer" , "Bottlenose creature?" .)