Attempt to contact the dead - assuming that is not main relative (5)
I believe the answer is:
niece
'relative' is the definition.
(type of relative)
'attempt to contact the dead assuming that is not main' is the wordplay.
I cannot quite see how this works, but
'that is' could be 'ie' (short for Latin 'id est', 'that is') and 'ie' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for niece that I've seen before include "Family girl" , "Family relation" , "A sibling's child" , "Criminal" , "[PRIEST'S] daughter?" .)