Where exhibits are of medium benefit? Not sure (6)
I believe the answer is:
museum
'where exhibits are' is the definition.
(museums have exhibits)
'medium benefit? not sure' is the wordplay.
'medium' becomes 'm' (as in UK clothing sizes).
'benefit?' becomes 'use' (I've seen this in another clue).
'not sure' becomes 'um' (both might be said when unsure about something).
'm'+'use'+'um'='MUSEUM'
'of' is the link.
(Other definitions for museum that I've seen before include "Building where objects of interest are displayed" , "Place to keep and exhibit objects of interest" , "educational facility?" , "Building exhibiting objects of scientific or historic interest" , "depository for historical objects" .)