Half of a fourth (6)
I believe the answer is:
eighth
'half of a fourth' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eighth that I've seen before include "Fraction, twelve and a half per cent" , "of an old Spanish piece" , "Half a quarter" , "Half a quater" , "12.5 per cent" .)