Rough sailor varies at sea (8)
I believe the answer is:
abrasive
'rough' is the definition.
(similar in meaning)
'sailor varies at sea' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'at sea' indicates anagramming the letters.
'varies' with letters rearranged gives 'rasive'.
'ab'+'rasive'='ABRASIVE'
(Other definitions for abrasive that I've seen before include "rubbing" , "Grinding substance" , "Rough polisher" , "Tending to grind down" , "smooth? The opposite" .)