Marvellous degree that's relative (7)
I believe the answer is:
grandma
'that's relative' is the definition.
(grandma is a kind of relative)
'marvellous degree' is the wordplay.
'marvellous' becomes 'grand' (I've seen this before).
'degree' becomes 'ma' (Master of Arts is a type of degree).
'grand'+'ma'='GRANDMA'
(Other definitions for grandma that I've seen before include "standing before Moses in painting" , "Female in the family" , "Drag man out to meet the old lady" , "Nanny" , "Elderly relative" .)