Jack goes in low -- it's magic (4)
I believe the answer is:
mojo
'magic' is the definition.
(I've seen this before)
'jack goes in low' is the wordplay.
'jack' becomes 'j' (playing card abbreviation).
'goes in' indicates putting letters inside.
'low' becomes 'moo' ('moo' can be a synonym of 'low').
'j' inserted inside 'moo' is 'MOJO'.
'it's' is the link.
(Other definitions for mojo that I've seen before include "American charm" , "Magical influence" , "Magical power" , "Magic charm or power in African-American culture, as heard in R&B music" .)