Building add-on (3)
I believe the answer is:
ell
'building add-on' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ell that I've seen before include "Measure, approx. fourty-five in." , "Tailor's measure" , "Once, forty-five inches" , "approximately 45 inches" , "Old measurement" .)