Fan of TV show from 2001 hiding in box? (7)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

K (abbreviation for kilo = 1000, used in scientific units and in quoting large sums of money) twice (so 2000) + I (Roman numeral for 1); all hiding in TREE (of which a box tree is an example; Eccles has helpfully included the question mark to indicate a definition by example). Slang for a fan of the TV show Star Trek . The answer was easy enough to guess from the crossers, but it took me ages to see the parsing.