Marine mammal mostly seen here (3)
I believe the answer is:
sea
'here' is the definition.
Both the answer and definition are adjectives. Perhaps there's an association between them I don't understand?
'marine mammal mostly' is the wordplay.
'marine mammal' becomes 'seal' (I've seen this before).
'mostly' means to remove the last letter.
'seal' with its last letter taken away is 'SEA'.
'seen' is the link.
(Other definitions for sea that I've seen before include "Watery expanse" , "70% of the earth's surface" , "Large body of salt water" , "7 [ship] s on this" , "Great expanse of water" .)