Colleagues single-handedly keep making ill-informed judgements (8)
I believe the answer is:
guessing
'making ill-informed judgements' is the definition.
'colleagues single-handedly keep' is the wordplay.
'keep' indicates a hidden word.
'GUESSING' can be found hidden inside 'colleagues single handedly'.
(Other definitions for guessing that I've seen before include "'Conjecturing, surmising (8)'" , "Speculating, estimating" , "Reckoning, estimating" , "Reckoning, hazarding" , "Supposing, estimating" .)