Taking off: one’s mass movement in art (13)
I believe the answer is:
impressionism
'art' is the definition.
(impressionism is a kind of art)
'taking off one's mass movement' is the wordplay.
I cannot really see how this works, but
'taking' could be 'on' (eg one taking drugs is 'on' drugs) and 'on' is present in the answer.
'one's' could be 'is' (Roman numeral) and 'is' is present in the answer.
'mass' could be 'press' (pressing is a kind of massing) and 'press' is found in the answer.
This explanation may well be incorrect...
'in' acts as a link.
Can you help me to learn more?
(Other definitions for impressionism that I've seen before include "art movement" , "Art school" .)