Initially told, then, to change a fraction (5)
I believe the answer is:
tenth
'a fraction' is the definition.
(tenth is a kind of fraction)
'initially told then to change' is the wordplay.
'initially' says to take the initial letters.
'to change' is an anagram indicator.
The initial letter of 'told' is 't'.
'then' with letters rearranged gives 'enth'.
't'+'enth'='TENTH'
(Other definitions for tenth that I've seen before include "after nine" , "(Decimal?) fraction" , "Fraction in tithe" , "Tithe proportion" , "0.1" .)