Fruit-tree thus gripped by curse (6)
I believe the answer is:
damson
'fruit-tree' is the definition.
'thus gripped by curse' is the wordplay.
'thus' becomes 'so' ('so' can be a synonym of 'thus').
'gripped by' is an insertion indicator.
'curse' becomes 'damn' ('damn' can be a synonym of 'curse').
'so' put into 'damn' is 'DAMSON'.
(Other definitions for damson that I've seen before include "Type of plum" , "Dark purple plum" , "Small purple plum" , "Small stone-fruit for nomads" , "might one be in a jam?" .)