Building style based on 500, or one plus 100 (5)
I believe the answer is:
doric
'building style' is the definition.
(architectural style)
'500 or one plus 100' is the wordplay.
'500' becomes 'D' (Roman numeral for 500).
'one' becomes 'i' (Roman numeral).
'plus' means one lot of letters go next to another.
'100' becomes 'C' (Roman numeral).
'd'+'or'+'i'+'c'='DORIC'
'based on' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for doric that I've seen before include "Greek dialect" , "Classical building style" , "Greek architectural order" , "hairdo tips costly, for the most part" , "In a classical Greek style of architecture" .)