Marginal changes may be frightening (8)
I believe the answer is:
alarming
'be frightening' is the definition.
(similar in meaning)
'marginal changes' is the wordplay.
'changes' is an anagram indicator.
'marginal' with letters rearranged gives 'ALARMING'.
'may' is the link.
(Other definitions for alarming that I've seen before include "Disturbing, frightening" , "Scary, frightening" , "Worrying or disturbing" , "Frightening in marginal way" , "Dreadful" .)