Room extension (3)
I believe the answer is:
ell
'room extension' 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 "Former measure of length" , "Measure, approx. fourty-five in." , "Former textile measure" , "Old unit of length" , "Once, forty-five inches" .)