Tate's opening week has Bridget Riley works in more than one section (3-4)
I believe the answer is:
two-part
'in more than one section' is the definition.
(in two sections or parts)
'tate's opening week has bridget riley works' is the wordplay.
'opening' indicates taking the first letters.
'week' becomes 'w' (abbreviation).
'has' is a charade indicator (letters next to each other).
'bridget riley works' becomes 'op art' (Bridget Riley was an op art artist).
The initial letter of 'tate' is 't'.
't'+'w'+'opart'='TWO-PART'
(Other definitions for two-part that I've seen before include "(Of music) composed for duet" , "Composed for a pair of voices" , "Consisting of a couple of sections" .)