Right-angled annex (3)
I believe the answer is:
ell
'right-angled annex' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ell that I've seen before include "Old textile measure" , "Old measure equivalent to about 45 inches in England" , "Measure, approx. fourty-five in." , "Former measure of length" , "Length of cloth" .)