A number could be seen outside this sporting fixture? (5)
I believe the answer is:
event
'this sporting fixture?' is the definition.
'event' can be an answer for 'fixture?' (I've seen this before). I'm not certain of the 'this sporting' bit.
'a number could be seen outside' is the wordplay.
'a number' becomes 'n' (n can represent any number).
'could' becomes 'evet' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'be seen outside' means one lot of letters goes inside another.
'n' placed into 'evet' is 'EVENT'.
(Other definitions for event that I've seen before include "Result; incident" , "one item in programme" , "celebration?" , "tournament" , "A thing taking place" .)