Mondrian's first to abrogate a religious genre (5)
I believe the answer is:
pieta
'a religious genre' is the definition.
(type of artwork depicting the Virgin Mary and Jesu)
'mondrian's first to abrogate' is the wordplay.
'mondrian' becomes 'Piet' (painter Piet Mondrian is an example).
'first to' indicates taking the first letters.
The initial letter of 'abrogate' is 'a'.
'piet'+'a'='PIETA'
(Other definitions for pieta that I've seen before include "mournful image" , "religious picture?" , "Sculpture of Mary with Christ's body" , "Religious depiction" , "Devotional painting" .)