Painting located in London museum? There you have it! (5)
I believe the answer is:
voila
'there you have it' is the definition.
(I've seen this before)
'painting located in london museum?' is the wordplay.
'painting' becomes 'oil' (oil painting is a kind of painting).
'located in' indicates putting letters inside.
'london museum?' becomes 'va' (V and A - Victoria and Albert Museum).
'oil' placed within 'va' is 'VOILA'.
(Other definitions for voila that I've seen before include "Announcement from French producer" , "French interjection - there you are!" , "The French see" , "See there, howzat - in France" , "See there, behold - in France" .)