Sport in Six Nations finally showing destructive influence (5)
I believe the answer is:
virus
'destructive influence' is the definition.
'virus' can be an answer for 'influence' (I have seen 'Corrupting influence' mean 'virus' so perhaps 'influence' could also mean 'virus'). I'm not certain of the 'destructive' bit.
'sport in six nations finally' is the wordplay.
'sport' becomes 'RU' (rugby union).
'in' is an insertion indicator.
'six' becomes 'VI' (Roman numerals).
'finally' suggests the final letters.
The last letter of 'nations' is 's'.
'vi'+'s'='vis'
'ru' inserted inside 'vis' is 'VIRUS'.
'showing' is the link.
Can you help me to learn more?
(Other definitions for virus that I've seen before include "Disease carrier or computer bug" , "horror on computer" , "Infectious particle" , "Bodily or computer infection" , "Cause of computer malfunctions?" .)