Marine mammal remarkably large: yes! (4,4)
I believe the answer is:
grey seal
'marine mammal' is the definition.
(I've seen this before)
'remarkably large yes' is the wordplay.
'remarkably' is an anagram indicator (I've seen this in other clues).
'large'+'yes'='largeyes'
'largeyes' with letters rearranged gives 'GREY SEAL'.
(Other definitions for grey seal that I've seen before include "Sea creature" , "Marine mammal" , "North Atlantic mammal" , "N. Atlantic mammal" .)