Sea creatures dashed at seamen (8)
I believe the answer is:
manatees
'sea creatures' is the definition.
(manatees are aquatic animals)
'dashed at seamen' is the wordplay.
'dashed' is an anagram indicator.
'at'+'seamen'='atseamen'
'atseamen' anagrammed gives 'MANATEES'.
(Other definitions for manatees that I've seen before include "Water creatures" , "Large plant-eating mammals of tropical waters" , "Men at sea (anag) - aquatic mammals" , "Sirenians" .)