Ultimately the full length less than four feet (3)
I believe the answer is:
ell
'less than four feet' is the definition.
(historical unit of measurement)
'ultimately the full length' is the wordplay.
'ultimately' says to take the final letters.
'length' becomes 'l'.
The final letters of 'the full' are 'el'.
'el'+'l'='ELL'
(Other definitions for ell that I've seen before include "45 inches (cloth)" , "Old measure equivalent to about 45 inches in England" , "Once, forty-five inches" , "Old 45-inch measure" , "Measure, approx. fourty-five in." .)