Great match, switching over at halfway (4)
I believe the answer is:
mega
'great' is the definition.
(old-fashioned UK slang for 'very good')
'match switching over at halfway' is the wordplay.
'match switching over' becomes 'meg' (I am not sure about this - if you are sure you should believe this answer much more).
'halfway' means to take half the letters.
'at' with half the letters taken is 'a'.
'meg'+'a'='MEGA'
Can you help me to learn more?
(Other definitions for mega that I've seen before include "Enormous; very good" , "Smashing" , "extremely good" , "wicked" , "Very large (colloq.)" .)