Call up Derek over purchases being returned (5)
I believe the answer is:
evoke
'call up' is the definition.
(I know that evoke is a more specific form of the action call up)
'derek over purchases being returned' is the wordplay.
'purchases' says the answer is hidden in the clue (I've seen 'purchasing' mean this).
'being returned' says the letters should be written in reverse.
'ekove' can be found hidden inside 'derek over'.
'ekove' backwards is 'EVOKE'.
(Other definitions for evoke that I've seen before include "Bring (a thing) to mind" , "Call to mind (memories)" , "Arouse" , "Conjure up" , "call-up" .)