Small auction item in assigned position (4)
I believe the answer is:
slot
'assigned position' is the definition.
(a slot is a position or place for something)
'small auction item' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'auction item' becomes 'lot' (a lot is an item for sale at auction).
's'+'lot'='SLOT'
'in' is the link.
(Other definitions for slot that I've seen before include "Space for coin or programme" , "Fit into puzzle" , "Scheduled time - hierarchical position" , "Long narrow slit" , "Long narrow aperture" .)