Attractive Exmoor heroine heading off, crazed (10)
I believe the answer is:
ornamental
'attractive' is the definition.
(ornamental things are pretty or attractive)
'exmoor heroine heading off crazed' is the wordplay.
'exmoor heroine' becomes 'lorna' (character in Lorna Doone: A Romance of Exmoor).
'heading off' means to remove the first letter.
'crazed' becomes 'mental' (mental can informally mean insane).
'lorna' with its initial letter taken off is 'orna'.
'orna'+'mental'='ORNAMENTAL'
(Other definitions for ornamental that I've seen before include "showy" , "Plant grown for attractive appearance" , "Decorative" , "non-functional" , "Fancy" .)