Gung-ho about (4)
I believe the answer is:
into
'gung-ho about' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for into that I've seen before include "Knowledgeable about" , "Keen on (informal)" , "To a point within - not I" , "Involved with" , "Keenly interested in" .)