Turn pale when division changes sides (6)
I believe the answer is:
blanch
'turn pale' is the definition.
(synonyms)
I don't understand the remainder of the clue.
(Other definitions for blanch that I've seen before include "Become white or pale, maybe with fright" , "Scald briefly, then drain" , "Remove the colour from something" , "Remove the colour, turn white" , "Go white, lose facial colour" .)