Heart expert necromancer boxes up (6)
I believe the answer is:
centre
'heart' is the definition.
(synonyms)
'expert necromancer boxes up' is the wordplay.
'boxes' indicates the answer is hidden within the clue (to box can mean to contain).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'ertnec' can be found hidden inside 'expert necromancer'.
'ertnec' written backwards gives 'CENTRE'.
(Other definitions for centre that I've seen before include "The point equidistant from everywhere" , "Place in the 18 down" , "'Things fall apart; the ... cannot hold' (Yeats)" , "The point equidistant from everything" , "See 7" .)