Guess a guru's out (5)
I believe the answer is:
augur
'guess' is the definition.
(I know that augur is a type of predict)
'a guru's out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'guru' with letters rearranged gives 'ugur'.
'a'+'ugur'='AUGUR'
(Other definitions for augur that I've seen before include "Be an omen (of) - Roman religious official" , "forecast" , "Divine" , "Prophesy" , "Bode, foretell" .)