Keep moving it in marble (7)
I believe the answer is:
agitate
'keep moving' is the definition.
(I've seen this before)
'it in marble' is the wordplay.
'in' means one lot of letters goes inside another.
'marble' becomes 'agate' (I've seen this before).
'it' placed into 'agate' is 'AGITATE'.
(Other definitions for agitate that I've seen before include "Disturb or shake" , "Campaign" , "Stir; arouse" , "Stir violently" , "Upset, disturb the surface of" .)