Beat the speed of light? Speed is boring! (8)
I believe the answer is:
chastise
'beat' is the definition.
(I've seen this before)
'the speed of light? speed is boring' is the wordplay.
'the speed of light?' becomes 'c' (I've seen this in other clues).
'speed' becomes 'haste' (haste is a kind of speed).
'boring' is an insertion indicator (some letters must 'bore' a hole into other letters).
'haste' enclosing 'is' is 'hastise'.
'c'+'hastise'='CHASTISE'
(Other definitions for chastise that I've seen before include "Whip perhaps" , "Punished severely" , "Inflict physical corrective treatment" , "Berate" , "Censure severely" .)