Mother possessed by a spirit that's wicked (7)
I believe the answer is:
amazing
'wicked' is the definition.
'mother possessed by a spirit' is the wordplay.
'mother' becomes 'ma' (ma is a kind of mother).
'possessed by' indicates putting letters inside.
'spirit' becomes 'zing' (I've seen this before**).
'a'+'zing'='azing'
'ma' put inside 'azing' is 'AMAZING'.
'that's' is the link.
(Other definitions for amazing that I've seen before include "Unbelievable!" , "Astounding, awe-inspiring" , "Hymn that 'sav'd a wretch like me'?" , "Stunning, surprising" , "Filling with wonder, a huge surprise" .)