It separates us from Britain (5,3)
I believe the answer is:
irish sea
'it separates us from britain' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for irish sea that I've seen before include "Water to west of Britain" , "It separates Ireland from England and Wales" , "Man's environment" , "Water west of Wales" , "Man's found here" .)