The "A" of MoMA (3)
I believe the answer is:
art
'the a of moma' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for art that I've seen before include "Work such as painting or sculpture" , "Works in a gallery" , "This gallery displays paintings" , "Creative works; skill" , "Painting, for instance" .)