Building extensions (4)
I believe the answer is:
ells
'building extensions' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ells that I've seen before include "Former length measures" , "Old English lengths" , "Old measures of length to sell" , "Extensions to buildings at right angles: archaic cloth measurements" , "Obsolete units of measurement" .)