Reject fiancée, essentially following incentive (5)
I believe the answer is:
spurn
'reject' is the definition.
(synonyms)
'fiancee essentially following incentive' is the wordplay.
'essentially' indicates the central letters.
'following' means one lot of letters go next to another.
'incentive' becomes 'spur' (spur can mean an incentive or stimulus).
The middle letter of 'fiancee' is 'n'.
'n' put after 'spur' is 'SPURN'.
(Other definitions for spurn that I've seen before include "Refuse with contempt" , "Contemptuously dismiss" , "discard" , "turn away" , "Reject disdainfully" .)