Yield in unprecedented fashion (4)
I believe the answer is:
cede
'yield' is the definition.
('cede' can be a synonym of 'yield')
'in unprecedented fashion' is the wordplay.
'in' indicates a hidden word.
'CEDE' is hidden within 'unprecedented fashion'.
(Other definitions for cede that I've seen before include "Yield formally" , "Surrender or yield" , "Surrender or grant" , "Transfer" , "21 across" .)