Society's overwhelmed by mounting box files (5)
I believe the answer is:
rasps
'files' is the definition.
(rasp is a kind of file)
'society's overwhelmed by mounting box' is the wordplay.
'society' becomes 's' (common abbreviation in names of organisations).
'overwhelmed by' indicates putting letters inside (I've seen this in other clues).
'mounting' shows that the letters should be reversed in order (going up in a down clue).
'box' becomes 'spar' (sparring is a kind of boxing).
'spar' in reverse letter order is 'raps'.
's' put into 'raps' is 'RASPS'.
(Other definitions for rasps that I've seen before include "Jars" , "fruit briefly" , "Grates harshly" , "speaks hoarsely" , "Coarse files" .)