Extremely fast hedgehog in game (5)
I believe the answer is:
sonic
'hedgehog in game' is the definition.
'sonic' can be an answer for 'hedgehog' (I've seen this before). I am not sure about the 'in game' bit.
'extremely fast' is the wordplay.
'extremely' becomes 'so' ('so' can mean 'extremely' or 'very').
'fast' becomes 'nic' (I can't justify this - if you can you should give a lot more credence to this answer).
'so'+'nic'='SONIC'
Can you help me to learn more?
(Other definitions for sonic that I've seen before include "Producing sound" , "Like mine, possibly" , "Type of boom" , "Travelling at Mach One" , "Sort of boom" .)