Keen about replacing motion, not second (4)
I believe the answer is:
into
'keen about' is the definition.
(I've seen this in another clue)
'replacing motion not second' is the wordplay.
'replacing' is an anagram indicator (re-placing: placing the letters again).
'not' indicates named letters should be taken away.
'second' becomes 'mo' (both are short periods of time).
'motion' with 'mo' taken away is 'tion'.
'tion' is an anagram of 'INTO'.
(Other definitions for into that I've seen before include "Fascinated by" , "Taking a lively and active interest in (colloq.)" , "Liking" , "Keenly interested in" , "Showing enthusiasm for" .)