Sport I abandoned that is extremely rough (6)
I believe the answer is:
rudest
'extremely rough' is the definition.
(rude can mean rough or basic)
'sport i abandoned that is' is the wordplay.
'sport' becomes 'RU' (rugby union).
'abandoned' is a deletion indicator.
'that is' becomes 'id est' ('that is' in Latin).
'idest' with 'i' taken away is 'dest'.
'ru'+'dest'='RUDEST'
(Other definitions for rudest that I've seen before include "Most bad mannered!" , "Least polite, most unmannerly" , "Most unrefined" , "Most vulgar" , "Most impolite" .)