Guess fool upset bird (6)
I believe the answer is:
assume
'guess' is the definition.
(I've seen this before)
'fool upset bird' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'upset' is a reversal indicator.
'bird' becomes 'emu' (emu is a type of bird).
'emu' written backwards gives 'ume'.
'ass'+'ume'='ASSUME'
(Other definitions for assume that I've seen before include "Guess" , "Suppose; put on" , "Take as true or take for granted" , "Take as read" , "Take on, affect" .)