Notices a different boater (8)
I believe the answer is:
canoeist
'boater' is the definition.
(canoeist is a kind of boater)
'notices a different' is the wordplay.
'different' is an anagram indicator (letters placed in a different order).
'notices'+'a'='noticesa'
'noticesa' is an anagram of 'CANOEIST'.
(Other definitions for canoeist that I've seen before include "sportsman" , "Paddler" , "Person who paddles a kayak or suchlike" , "Boat user" , "rower" .)