Estimate Greek initially uses incorrectly (5)
I believe the answer is:
guess
'estimate' is the definition.
(I know that estimate can be written as guess)
'greek initially uses incorrectly' is the wordplay.
'initially' says to take the initial letters.
'incorrectly' indicates anagramming the letters (I've seen this in other clues).
The first letter of 'greek' is 'g'.
'uses' is an anagram of 'uess'.
'g'+'uess'='GUESS'
(Other definitions for guess that I've seen before include "Divine - estimate" , "Speculation" , "Reckon" , "Theory" , "Estimate on the basis of little information" .)