Bishops and knights, say, in carefully planned moves (3,6)
I believe the answer is:
set pieces
'carefully planned moves' is the definition.
Although both the answer and definition are plural nouns, I don't see how one could define the other.
'bishops and knights say' is the wordplay.
'bishops' becomes 'set' (I can't justify this - if you can you should give a lot more credence to this answer).
'and' says to put letters next to each other.
'knights say' becomes 'pieces' (knight is a kind of piece).
'set'+'pieces'='SET PIECES'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for set pieces that I've seen before include "rehearsed moves" , "fireworks displays" , "Elaborate formal speeches" , "men on chess board?" , "bishops and others?" .)