Small objects, not medium in places (5)
I believe the answer is:
sites
'places' is the definition.
(site is a kind of place)
'small objects not medium' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'objects' becomes 'items'.
'not' is a deletion indicator.
'medium' becomes 'm' (as in UK clothing sizes).
'items' with 'm' taken out is 'ites'.
's'+'ites'='SITES'
'in' acts as a link.
(Other definitions for sites that I've seen before include "Is set among building locations" , "places" , "internet locations" , "Spots" , "Locations or positions, as for building" .)