Modern in only one way (3)
I believe the answer is:
new
'modern' is the definition.
('new' is associated in meaning with 'modern')
'in only one way' is the wordplay.
'in' says the answer is hidden in the clue.
'NEW' can be found hidden inside 'only one way'.
(Other definitions for new that I've seen before include "Modern" , "Unused or novel" , "Unfamiliar, restored" , "Not previously used" , "Seen for the first time" .)