London art museum (4)
I believe the answer is:
tate
'london art museum' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tate that I've seen before include "Sugar merchant and art gallery philanthropist, founder, d. 1899" , "Britain/Modern gallery" , "famous art gallery beside London's Thames" , "Art gallery benefactor" , "One of four art galleries in England" .)