All settees, when rearranged, fit together (10)
I believe the answer is:
tessellate
'fit together' is the definition.
(tessellate means to fit together without gaps)
'all settees when rearranged' is the wordplay.
'when rearranged' indicates anagramming the letters.
'all'+'settees'='allsettees'
'allsettees' anagrammed gives 'TESSELLATE'.
(Other definitions for tessellate that I've seen before include "marked in little squarish areas" , "perfectly fit to fill a space" , "Create mosaic" , "Form a mosaic - sell estate (anag)" , "Decorate with mosaics" .)