Berry lost by mistake (3)
I believe the answer is:
err
'mistake' is the definition.
(to err is to make a mistake or blunder)
'berry lost by' is the wordplay.
'lost' suggests deleting specific letters.
'berry' with 'by' removed is 'ERR'.
(Other definitions for err that I've seen before include "Blunder" , "'to... is human,to forgive divine'" , "Deviate" , "Stray from the straight and narrow" , "Be inaccurate" .)