Measurement in the company of about 500 (5)
I believe the answer is:
width
'measurement' is the definition.
(width is a kind of measurement)
'in the company of about 500' is the wordplay.
'in the company of' becomes 'with' (I've seen this before).
'about' means one lot of letters goes inside another.
'500' becomes 'D' (Roman numeral for 500).
'with' placed around 'd' is 'WIDTH'.
(Other definitions for width that I've seen before include "Measurement across" , "Diameter" , "compass" , "Range" , "Lateral measurement" .)