Class is reversing positions (5)
I believe the answer is:
sites
'positions' is the definition.
(I know this)
'class is reversing' is the wordplay.
'class' becomes 'set' (I've seen this before).
'reversing' says the letters should be written in reverse.
'set'+'is'='setis'
'setis' in reverse letter order is 'SITES'.
(Other definitions for sites that I've seen before include "Spots" , "Locations or positions, as for building" , "places" , "Is set among building locations" , "internet locations" .)