It's often abused by grocers dealing with multiples (10)
I believe the answer is:
apostrophe
'it's often abused by grocers' is the definition.
(reference to the grocers' apostrophe )
I cannot understand how the rest of the clue works.
(Other definitions for apostrophe that I've seen before include "Passage addressed to character" , "Letter omission indicator" , "Mark in exam's" , "It may be superfluous on tradesman's sign" , "At which point discarded letters" .)