Length of enclosure (4)
I believe the answer is:
yard
I believe this is a double definition.
'length' is the first definition.
(I have seen 'A length ' mean 'yard' so perhaps 'length' could also mean 'yard')
'enclosure' is the second definition.
(I know that yard is a type of enclosure)
'of' is the link.
(Other definitions for yard that I've seen before include "Sail support" , "Thirty six inches" , "Dray (anag.)" , "Enclosed area; length" , "Imperial unit of length" .)