Action during championships must get evidence of ownership (5,5)
I believe the answer is:
title deeds
'evidence of ownership' is the definition.
(title deeds prove ownership)
'action during championships' is the wordplay.
'action' becomes 'deed' (deed is a kind of action).
'during' means one lot of letters goes inside another.
'championships' becomes 'titles' ('title' can be a synonym of 'championship').
'deed' placed within 'titles' is 'TITLE-DEEDS'.
'must get' is the link.
(Other definitions for title deeds that I've seen before include "describing the Acts of the Apostles, for example?" , "Homely documents" , "Legal documents" .)