Measure height missed in the lower regions (3)
I believe the answer is:
ell
'measure' is the definition.
(historical unit of length)
'height missed in the lower regions' is the wordplay.
'height' becomes 'h' (abbreviation in geometry).
'missed in' indicates named letters should be taken away.
'the lower regions' becomes 'hell'.
'hell' with 'h' taken out is 'ELL'.
(Other definitions for ell that I've seen before include "45 inches (cloth)" , "Measure, approx. fourty-five in." , "Old measure equivalent to about 45 inches in England" , "Old cloth measure" , "Extension" .)