Source of light a big hit in space (3,4)
I believe the answer is:
gas lamp
'source of light' is the definition.
(I've seen this before)
'a big hit in space' is the wordplay.
'a big hit' becomes 'slam' (to slam is to hit hard).
'in' is an insertion indicator.
'space' becomes 'gap' (gap is a kind of space).
'slam' going inside 'gap' is 'GAS-LAMP'.
(Other definitions for gas lamp that I've seen before include "Burning source of light" , "Source of illumination" , "Victorian street light" , "Pre-electric light" , "may be illuminating" .)