Is and isn't one way to be emphatic (6)
I believe the answer is:
insist
'be emphatic' is the definition.
(I've seen this in another clue)
'is and isn't one way' is the wordplay.
'and' says to put letters next to each other.
'one way' is an anagram indicator (another way of arranging the letters).
'is' put after 'isnt' is 'isntis'.
'isntis' is an anagram of 'INSIST'.
'to' acts as a link.
(Other definitions for insist that I've seen before include "Persist in pressing" , "Assert firmly" , "express convinced viewpoint" , "Press" , "Demand persistently" .)