Angled annex (3)
I believe the answer is:
ell
'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 unit of length" , "approximately 45 inches" , "Old measure equivalent to about 45 inches in England" , "Former measure of length" , "railway in Chicago?" .)