500 plus surrounding span (5)
I believe the answer is:
width
'span' is the definition.
(I've seen this in another clue)
'500 plus surrounding' is the wordplay.
'500' becomes 'D' (Roman numeral for 500).
'plus' becomes 'with' (I've seen this in another clue).
'surrounding' indicates putting letters inside.
'd' put into 'with' is 'WIDTH'.
(Other definitions for width that I've seen before include ""Never mind the quality, feel the ___"" , "Extent from side to side" , "Thickness" , "Distance from side to side" , "Sideways extent" .)