About to rest, change striker for amateur game (7,5)
I believe the answer is:
'amateur game' is the definition.
(Oxford and Cambridge university rugby match)
'about to rest change striker' is the wordplay.
'about' indicates putting letters inside.
'to rest' becomes 'sit' (I've seen this before).
'change' becomes 'vary' (synonyms).
'striker' becomes 'match' (a match can be struck).
'sit' inserted into 'vary' is 'varsity'.
'for' acts as a link.
(Other definitions for varsity match that I've seen before include "Eg, Oxford v. Cambridge", "Eg, Oxford/Cambridge game".)