Kind of light? Extremely! (4)
I believe the answer is:
very
I believe this clue is a triple definition!
'kind of' is the first definition.
(thesaurus)
'light?' is the second definition.
(I've seen this before)
'extremely' is the third definition.
(I've seen this in another clue)
(Other definitions for very that I've seen before include "To a high degree or extent" , "Kind of light" , "Frightfully" , "type of light" , "Exactly, really" .)