Jack entering low spell (4)
I believe the answer is:
'spell' is the definition.
(mojo can mean a spell or charm)
'jack entering low' is the wordplay.
'jack' becomes 'J' (playing card abbreviation).
'entering' indicates putting letters inside.
'low' becomes 'moo' (both can mean to make a cow noise).
'j' put within 'moo' is 'MOJO'.
(Other definitions for mojo that I've seen before include "Magical influence" , "Magic charm or power in African-American culture, as heard in R&B music" , "American charm" , "Magical power" .)