Close up by one concerning marine mammal (3,4)
I believe the answer is:
sea lion
'marine mammal' is the definition.
(I've seen this before)
'close up by one concerning' is the wordplay.
'close up' becomes 'seal' (I've seen this before).
'by' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'concerning' becomes 'on'.
'seal'+'i'+'on'='SEA-LION'
(Other definitions for sea lion that I've seen before include "Large-eared marine mammal, sometimes performing" , "Marine beast, the male having a mane" , "Aquatic mammal - code name for planned German invasion of England, 1940" , "Acrobatic aquatic mammal" , "Creature" .)