Estimating (8)
I believe the answer is:
guessing
'estimating' is the definition.
('guess' can be a synonym of 'estimate')
This is the entire clue.
(Other definitions for guessing that I've seen before include "Reckoning, estimating" , "Supposing, estimating" , "'Conjecturing, surmising (8)'" , "Speculating, estimating" , "Reckoning, hazarding" .)