Boot a grounder, say (3)
I believe the answer is:
err
'boot a grounder say' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for err that I've seen before include "Make mistakes" , "Make a wrong move" , "'to... is human,to forgive divine'" , "Be inaccurate" , "Be mistaken" .)