Hedgehog thus keeps running (5)
I believe the answer is:
sonic
'hedgehog' is the definition.
(I've seen this before)
'thus keeps running' is the wordplay.
'thus' becomes 'sic' ('thus' in Latin).
'keeps' means one lot of letters goes inside another.
'running' becomes 'on' (eg when a machine is running it is on).
'sic' enclosing 'on' is 'SONIC'.
(Other definitions for sonic that I've seen before include "Hedgehog" , "Involving sound waves, say a boom" , "Sort of boom" , "Employing sound" , "of noise" .)