Three score and ten sides out for a sporting occasion (5)
I believe the answer is:
event
'a sporting occasion' is the definition.
(eg athletics events)
'three score and ten sides out' is the wordplay.
'three score and ten' becomes 'seventy' (score can mean twenty).
'sides out' means to look at the middle letters (letters at the sides are removed).
The centre of 'seventy' is 'EVENT'.
'for' is the link.
(Other definitions for event that I've seen before include "contest in programme" , "'Occurrence, happening (5)'" , "ride competitively" , "Race contest; occasion" , "Significant happening" .)