Crew that rows hard placing in the top ten (6)
I believe the answer is:
eighth
'placing in the top ten' is the definition.
(eighth place is within the top 10)
'crew that rows hard' is the wordplay.
'crew that rows' becomes 'eight' (crew of eight rowers).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'eight'+'h'='EIGHTH'
(Other definitions for eighth that I've seen before include "Real fraction" , "Day of Christmas to receive maids a-milking" , "Twelve and a half percent" , "Fraction, twelve and a half per cent" , "Half a quarter" .)