Groundsman initially uses wrong estimate (5)
I believe the answer is:
guess
'estimate' is the definition.
(I know that guess is a type of estimate)
'groundsman initially uses wrong' is the wordplay.
'initially' suggests taking the first letters.
'wrong' indicates anagramming the letters.
The initial letter of 'groundsman' is 'g'.
'uses' anagrammed gives 'uess'.
'g'+'uess'='GUESS'
(Other definitions for guess that I've seen before include "Divine - estimate" , "Dare say" , "Theory" , "the Americans believe it" , "Estimate; suppose" .)