Speed of light has mass in the void (5)
I believe the answer is:
chasm
'the void' is the definition.
(both can mean a large gulf or gap)
'speed of light has mass' is the wordplay.
'speed of light' becomes 'c' (as in E=mc2).
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'c'+'has'+'m'='CHASM'
'in' acts as a link.
(Other definitions for chasm that I've seen before include "One gapes" , "Yawning gap" , "Gaping hollow" , "What's split" , "Deep opening in the earth's surface" .)