Gridiron units (5)
I believe the answer is:
yards
'gridiron units' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for yards that I've seen before include "Three feet in each" , "Pieces of uncultivated ground adjoining buildings" , "Courts" , "belonging to investigators" , "Measures; enclosures" .)