Enthusiastic about one -- not excited (4)
I believe the answer is:
into
'enthusiastic about' is the definition.
(I've seen this before)
'one not excited' is the wordplay.
'one' becomes 'i' (Roman numeral).
'excited' indicates an anagram.
'not' with letters rearranged gives 'nto'.
'i'+'nto'='INTO'
(Other definitions for into that I've seen before include "Enthusiastic about" , "Keen on (informal)" , "Involved with" , "Curious about" , "Teachers' trade union" .)