Enthusiastic about switching it on (4)
I believe the answer is:
into
'enthusiastic about' is the definition.
(into can mean interested in or enthusiastic about)
'switching it on' is the wordplay.
'switching' indicates anagramming the letters.
'it'+'on'='iton'
'iton' is an anagram of 'INTO'.
(Other definitions for into that I've seen before include "To a position within" , "Taking an active and lively interest (in something)" , "Expression of division" , "Curious about" , "Taking a lively and active interest in (colloq.)" .)