Reason a follower has not changed (5)
I believe the answer is:
basis
'reason' is the definition.
(I've seen this before)
'a follower has not changed' is the wordplay.
'a follower' becomes 'b' ('b' is the follower of 'a' in the alphabet).
'has' says to put letters next to each other.
'not changed' becomes 'asis' (I've seen this before).
'b'+'asis'='BASIS'
(Other definitions for basis that I've seen before include "Reason" , "Principle on which a theory is founded" , "First principle" , "Foundation of a theory or process" , "Underlying support (for an idea)" .)