Mariner's fears are misplaced (8)
I believe the answer is:
seafarer
'mariner's' is the definition.
('seafarer' can be a synonym of 'mariner')
'fears are misplaced' is the wordplay.
'misplaced' is an anagram indicator.
'fears'+'are'='fearsare'
'fearsare' anagrammed gives 'SEAFARER'.
(Other definitions for seafarer that I've seen before include "Sinbad?" , "Traveller over water" , "A traveller on the ocean, like sailor" , "One going by ship" , "One on the water" .)