Big contract to fill hole in light (3,4)
I believe the answer is:
gas lamp
'light' is the definition.
(gas lamp is a kind of light)
'big contract to fill hole' is the wordplay.
'big contract' becomes 'slam' (I've seen this before).
'to fill' indicates putting letters inside.
'hole' becomes 'gap'.
'slam' put inside 'gap' is 'GAS-LAMP'.
'in' is the link.
(Other definitions for gas lamp that I've seen before include "Burning source of light" , "Sort of light" , "lighter" , "may be illuminating" , "Form of illumination" .)