Extremely light (4)
I believe the answer is:
very
I believe this is a double definition.
'extremely' is the first definition.
(very can mean extremely or considerably)
'light' is the second definition.
(I've seen this before)
(Other definitions for very that I've seen before include "type of light" , "Extremely - light" , "Exceedingly" , "Frightfully" , "jolly British!" .)