Perhaps good in a sport, showing skill (8)
I believe the answer is:
arguably
'perhaps' is the definition.
(both can introduce a proposition or claim)
'good in a sport showing skill' is the wordplay.
'good' becomes 'g' (abbreviation).
'in' is an insertion indicator.
'sport' becomes 'RU' (rugby union).
'showing skill' becomes 'ably' (able can mean skilled).
'a'+'ru'='aru'
'g' placed within 'aru' is 'argu'.
'argu'+'ably'='ARGUABLY'
(Other definitions for arguably that I've seen before include "as may be proved" , "One could claim" .)