Corrupting influence in Rugby Union, in Six Nations, ultimately (5)
I believe the answer is:
virus
'corrupting influence' is the definition.
(I've seen this before)
'rugby union in six nations ultimately' is the wordplay.
'rugby union' becomes 'ru'.
'in' is an insertion indicator.
'six' becomes 'vi' (roman numeral).
'ultimately' suggests the final letters.
The final letter of 'nations' is 's'.
'vi'+'s'='vis'
'ru' going within 'vis' is 'VIRUS'.
'in' acts as a link.
(Other definitions for virus that I've seen before include "What may kill your computer" , "Organism causing disease, or malicious program in computer" , "Form of infection" , "Infectious particle" , "Cause of computer malfunctions?" .)