Good: Sue's revised estimate (5)
I believe the answer is:
guess
'estimate' is the definition.
(I know that estimate can be written as guess)
'good sue's revised' is the wordplay.
'good' becomes 'g' (abbreviation).
'revised' is an anagram indicator.
'sues' is an anagram of 'uess'.
'g'+'uess'='GUESS'
(Other definitions for guess that I've seen before include "Hypothesis" , "Take a stab at the answer" , "Dare say" , "the Americans believe it" , "Speculate" .)