Fly from assignation (4)
I believe the answer is:
gnat
'fly' is the definition.
(type of insect)
'from assignation' is the wordplay.
'from' indicates the answer is hidden within the clue.
'GNAT' is hidden in the letters of 'assignation'.
(Other definitions for gnat that I've seen before include "Stinging insect" , "Small fly that bites" , "Small biter" , "Sucker (if female)" , "little irritation" .)