Waterproof one working with marine mammal (3,4)
I believe the answer is:
sea lion
'marine mammal' is the definition.
(I've seen this before)
'waterproof one working' is the wordplay.
'waterproof' becomes 'seal' (I've seen this in another clue).
'one' becomes 'i' (Roman numeral).
'working' becomes 'on' (e.g. a machine is working or on).
'seal'+'i'+'on'='SEA-LION'
'with' is the link.
(Other definitions for sea lion that I've seen before include "Large marine mammal" , "Acrobatic aquatic mammal" , "Aquatic mammal - code name for planned German invasion of England, 1940" , "Marine animal, used as a circus performer" , "Large-eared mammal of Pacific Ocean, like Leo" .)