Answer involving a repeat fixture (6)
I believe the answer is:
replay
'repeat fixture' is the definition.
(I've seen this before)
'answer involving a' is the wordplay.
'answer' becomes 'reply' ('reply' can be a synonym of 'answer').
'involving' means one lot of letters goes inside another.
'reply' going around 'a' is 'REPLAY'.
(Other definitions for replay that I've seen before include "Second showing?" , "another game" , "Hold match again" , "A restaged match" , "second game" .)