Artist twice entering dock (6)
I believe the answer is:
cubist
'artist' is the definition.
(cubist is a kind of artist)
'twice entering dock' is the wordplay.
'twice' becomes 'bis' (I've seen this in other clues).
'entering' means one lot of letters goes inside another.
'dock' becomes 'cut'.
'bis' going inside 'cut' is 'CUBIST'.
(Other definitions for cubist that I've seen before include "Picasso, e.g." , "Painter like Picasso" , "Painter in a style using geometric shapes" , "Braque or Picasso, for example" , "Geometrical painter" .)