It's becoming a type of fraction (6)
I believe the answer is:
proper
I believe this clue is a double definition.
'it's becoming' is the first definition.
(similar in meaning)
'a type of fraction' is the second definition.
(Other definitions for proper that I've seen before include "Seemly" , "Right, correct or appropriate" , "Befitting" , "Genuine; suitable" , "Genuine; part of church service" .)