Sport, and one reason it's dire (7)
I believe the answer is:
ruinous
'dire' is the definition.
(thesaurus)
'sport and one reason' is the wordplay.
'sport' becomes 'ru' (rugby union).
'and' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'reason' becomes 'nous'.
'ru'+'i'+'nous'='RUINOUS'
'it's' is the link.
(Other definitions for ruinous that I've seen before include "causing harm?" , "Destructive and causing downfall" , "Destructive; dilapidated" , "Causing destruction" , "Exorbitant" .)