Guess fool upset bird missing flight (6)
I believe the answer is:
assume
'guess fool' is the definition.
'assume' can be an answer for 'guess' (I've seen this before). I am unsure of the 'fool' bit.
'upset bird missing flight' is the wordplay.
'upset' says the letters should be written backwards.
'bird' becomes 'emu' (emu is a type of bird).
'missing flight' becomes 'ssa' (I am not sure about this - if you are sure you should believe this answer much more).
'emu'+'ssa'='emussa'
'emussa' reversed gives 'ASSUME'.
Can you help me to learn more?
(Other definitions for assume that I've seen before include "Accept as true without proof" , "Take to be the case" , "Guess" , "Take as read" , "Suppose; put on" .)