Twelve and a half percent (6)
I believe the answer is:
eighth
'twelve and a half percent' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for eighth that I've seen before include "The last Henry or Edward?" , "Half a quater" , "12.5 per cent" , "of an old Spanish piece" , "Fractional amount of drug" .)