Add-on (3)
I believe the answer is:
ell
'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 "45 inches (cloth)" , "Old cloth measure" , "distance" , "Measure, approx. fourty-five in." , "Embellish with old measure of length" .)